Le carrelage de la cuisine est encore mouillé.

Carrelage is a collective noun meaning tiling, tilework, or a tiled surface/floor.

A useful contrast:

• un carreau = one tile
• le carrelage = the tiled covering as a whole

So this sentence is not talking about one tile, but about the tiled surface.

Usually, yes.

In everyday French, le carrelage de la cuisine will often be understood as the kitchen tiles / the tiled kitchen floor. But literally, carrelage can also refer to tiling on a wall or another tiled surface.

So the most natural image is probably the kitchen floor is still wet, but the word itself is a little broader than that.

French often uses de + noun to show what something belongs to or which one it is.

So:

• le carrelage de la cuisine = literally the tiling of the kitchen

In English, we often turn this into a noun modifier:

• the kitchen tiles
• the kitchen floor

French usually prefers the de structure here.

Because cuisine is a feminine singular noun.

• de + la cuisine → de la cuisine
• If the noun were masculine singular with le, you would often get du

For example:

• le carrelage du salon = the living-room tiles/floor
• le carrelage de la cuisine = the kitchen tiles/floor

So the form depends on the gender of the noun that follows.

Because the subject is already clearly named: Le carrelage de la cuisine.

French normally uses:

• X est + adjective

So:

• Le carrelage de la cuisine est encore mouillé.

C'est is more common when:

• identifying something: C'est la cuisine
• making a general comment: C'est mouillé
• introducing a noun phrase: C'est un problème

With a specific noun subject plus an adjective, est is the normal choice.

Here, encore means still.

So the idea is:

• it was wet before
• and it has not dried yet

Encore can also mean again in other contexts, but in this sentence still is the natural meaning.

Because mouillé is an adjective, and in French it agrees with the noun it describes.

Here, it describes:

• le carrelage = masculine singular

So the adjective is:

• mouillé = masculine singular

Other forms would be:

• mouillée = feminine singular
• mouillés = masculine plural
• mouillées = feminine plural

For example:

• Les dalles sont mouillées. = The slabs/tiles are wet.

Yes, absolutely.

The difference is:

• le sol = the floor in general
• le carrelage = the tiling / tiled floor specifically

So:

• Le sol de la cuisine est encore mouillé = the kitchen floor is still wet
• Le carrelage de la cuisine est encore mouillé = the kitchen tiles/tiled floor are still wet

Use carrelage when you want to emphasize that the floor is tiled.

They are similar, but not identical.

• mouillé = wet
• humide = damp / humid / moist

Mouillé usually suggests a more obvious, direct wetness, like water on the surface.
Humide is weaker and can sound less immediate.

For a floor that has just been cleaned or spilled on, mouillé is the more natural word.

They are close, but not quite the same.

• le carrelage de la cuisine = the kitchen tiling / the tiles of the kitchen
• le carrelage dans la cuisine = the tiling in the kitchen

De la cuisine identifies which tiling you mean.
Dans la cuisine emphasizes where it is.

In this sentence, de la cuisine is the more natural phrasing if you mean the kitchen floor/tiles as a known part of the house.