A operadora tem o plano pré-pago.

Because operadora is a feminine singular noun, so it takes the feminine singular definite article a.

• a operadora = the operator / the carrier
• If it were masculine, you would expect o instead.

In Brazilian Portuguese, article + noun is very common, and using the article often sounds more natural than leaving it out.

In Brazil, operadora very often means a phone carrier / mobile provider / telecom company. In other contexts, it can also mean a female operator, depending on the situation.

In this sentence, because of plano pré-pago, learners will usually understand operadora as a carrier/company, not a person.

Tem is the third-person singular form of ter in the present tense:

• eu tenho
• você / ele / ela tem
• nós temos
• eles / elas têm

Since a operadora is singular, the correct form is tem.

The form têm with a circumflex is plural:

• As operadoras têm o plano pré-pago.

O is the masculine singular definite article, because plano is a masculine noun.

So:

• o plano = the plan
• um plano = a plan

Using o plano pré-pago makes it sound like a specific prepaid plan is being referred to. If you said um plano pré-pago, it would sound less specific.

Yes. That is also grammatical, but the meaning changes slightly.

• A operadora tem o plano pré-pago = the sentence points to a specific prepaid plan, or one already known in context.
• A operadora tem um plano pré-pago = the company has a prepaid plan, not necessarily one already identified.

In real-life Brazilian Portuguese, um is often more natural if you are introducing the plan for the first time.

Yes. Pré-pago is an adjective meaning prepaid.

It describes plano:

• plano pré-pago = prepaid plan

Because plano is masculine singular, the adjective appears in the masculine singular form pré-pago.

If the noun were feminine, the adjective would normally change:

• conta pré-paga = prepaid account / bill

The accent in pré shows the stressed vowel and is part of the correct spelling.

The hyphen in pré-pago is standard Portuguese spelling for this compound form. You should learn it as a set expression:

• pré-pago
• pós-pago

These are very common in Brazil when talking about phone plans.

A simple approximate pronunciation is:

• pré ≈ preh
• pago ≈ PAH-goo in many Brazilian accents, though the final o is often weaker than English speakers expect

So the whole phrase sounds roughly like PREH-PAH-goo.

Also:

• the stress in pré-pago falls strongly on pré
• in operadora, the stress is on do: o-pe-ra-DO-ra

Literally, ter means to have, but in everyday Portuguese it can also be used in a broader sense like to have available, to carry, or to offer, depending on context.

So in this sentence, tem can suggest:

• the carrier has a prepaid plan
• the carrier offers a prepaid plan
• the carrier provides a prepaid plan option

A more explicit business-style verb would be oferece:

• A operadora oferece um plano pré-pago.

Yes, it is grammatical and understandable. But depending on context, some Brazilians might more naturally say:

• A operadora tem plano pré-pago.
• A operadora oferece plano pré-pago.
• A operadora oferece um plano pré-pago.

Why? Because when speaking generally about services, Portuguese often omits the article before the product type:

• tem plano pré-pago = has prepaid plans / offers prepaid service

Your original sentence is still fine, especially if a particular plan has already been mentioned.

These are opposite terms commonly used for mobile service in Brazil:

• pré-pago = prepaid
  You pay first and then use the service.
• pós-pago = postpaid
  You use the service first and pay later, usually with a monthly bill.

So learners in Brazil will see both expressions very often:

• plano pré-pago
• plano pós-pago

The structure is:

article + noun + verb + article + noun + adjective

More specifically:

• A operadora = subject
• tem = verb
• o plano pré-pago = direct object

So the sentence follows the normal Portuguese word order: Subject + Verb + Object

This is the same basic order English often uses, which makes it relatively easy for English speakers to recognize.