Au magasin, je préfère ceux en coton, mais ma sœur choisit ceux en laine.

Au magasin means at the store or in the store, depending on context. French often uses à + le = au before masculine singular nouns.

• à = at / to / in
• le magasin = the store
• au magasin = at the store / in the store

Putting it at the beginning gives a setting for the whole sentence: At the store, ...

French often moves place expressions to the front like this for emphasis or to set the scene.

Ceux means those ones or simply those. It is a demonstrative pronoun, used to replace a masculine plural noun that has already been mentioned or is understood from context.

For example, if you are talking about pulls or vêtements:

• ceux en coton = the ones in cotton / the cotton ones
• ceux en laine = the ones in wool / the wool ones

So ceux avoids repeating the noun.

Because ceux points to specific things: those ones.

• ils means they, so it is a subject pronoun, not the right kind of word here.
• les means them or the, but it does not by itself mean those ones.
• ceux specifically replaces a plural masculine noun and means those ones / the ones.

Compare:

• Je préfère ceux en coton. = I prefer the cotton ones.
• Je les préfère. = I prefer them.
  This is grammatical, but it does not include the idea of the ones in cotton.

With materials in French, en is commonly used to mean made of or in a certain material.

So:

• en coton = made of cotton
• en laine = made of wool
• en bois = made of wood

That is why ceux en coton means the ones made of cotton.

Using de here would usually sound unnatural in this context.

Because ceux already replaces the noun. This is very common in French.

Instead of saying:

• je préfère les pulls en coton

you can say:

• je préfère ceux en coton

Here, ceux stands in for pulls or whatever plural masculine item is being discussed.

English does the same thing:

• I prefer the cotton ones.

French demonstrative pronouns change according to gender and number:

• celui = masculine singular
• celle = feminine singular
• ceux = masculine plural
• celles = feminine plural

So ceux is used when the hidden noun is:

1. masculine, and
2. plural

Example:

• les pulls → ceux
• les chemises → celles

So if the speaker were talking about feminine plural items, the sentence would use celles, not ceux.

They are two different verbs, each conjugated in the present tense:

• préférer → je préfère = I prefer
• choisir → elle choisit = she chooses

The subject changes too:

• je = I
• ma sœur = my sister = she

So:

• je préfère
• ma sœur choisit

This is just normal present-tense conjugation for two different verbs.

The infinitive is préférer, but in some present-tense forms the é in the stem changes to è.

So:

• je préfère
• tu préfères
• il/elle préfère
• ils/elles préfèrent

But:

• nous préférons
• vous préférez

This is a common spelling pattern in French verbs like préférer.

The first comma separates the introductory phrase Au magasin from the main clause.

• Au magasin, je préfère ceux en coton...

This is similar to English:

• At the store, I prefer the cotton ones...

The comma before mais is also normal because mais means but and joins two contrasting ideas:

• I prefer the cotton ones, but my sister chooses the wool ones.

So the punctuation helps show the structure clearly.

Yes. Both are possible, depending on how natural you want the English to sound.

• ceux en coton = those made of cotton
• ceux en coton = the cotton ones

The second version often sounds more natural in everyday English, but both reflect the French structure well.

Yes, in many contexts.

• je préfère = I prefer
• j’aime mieux = literally I like better, but often used to mean I prefer

So:

• Je préfère ceux en coton.
• J’aime mieux ceux en coton.

Both are understandable. Préférer is a direct, standard way to say to prefer.

Ceux is pronounced roughly like suh with the French vowel eu, not like English soo or see.

A simple approximation for English speakers is:

• ceux ≈ suh / sir without the r

Also remember:

• the x is silent

So in the sentence:

• je préfère ceux en coton the word ceux is short and smooth, with no strong final consonant sound.