È stata una decisione intelligente.

Because è is the verb form is. The accent distinguishes it from e, which means and.

In standard Italian, the accent is kept even at the beginning of a sentence and even on a capital letter, so È is correct.

È stata is the passato prossimo of essere.

It is formed with:

• the present tense of the auxiliary essere: è
• the past participle: stata

So this is the normal spoken way to say that something was in the past.

That is just how Italian works. The verb essere forms its compound tenses with the auxiliary essere plus its past participle stato/stata.

For example:

• sono stato/stata
• sei stato/stata
• è stato/stata

It may feel strange at first, but it is completely normal.

Because stata is feminine singular, and with essere, the past participle agrees in gender and number.

Here the relevant noun is una decisione intelligente, which is feminine singular, so the participle is stata.

Compare:

• È stato un errore. → masculine singular
• È stata una buona idea. → feminine singular

Italian often does not use a dummy subject like English it.

English says It was ..., but Italian can simply say È stata ... without adding a subject pronoun. The sentence is natural as it is, and the context usually makes clear what is being evaluated.

Because decisione is a singular countable noun, and Italian normally uses an article with nouns like this.

So:

• una decisione = an/a decision

Leaving out una here would sound wrong in standard Italian.

You can see it from the article una, and decisione is indeed a feminine noun.

A useful pattern is that many Italian nouns ending in -ione are feminine:

• la decisione
• la situazione
• la nazione

Like most patterns, this is very helpful but not something to trust blindly 100% without checking.

Because in Italian, descriptive adjectives often come after the noun.

So una decisione intelligente is the most natural neutral order.

You can sometimes put the adjective before the noun:

• un'intelligente decisione

But that sounds more literary, formal, or stylistically marked. For everyday Italian, una decisione intelligente is the safer choice.

Because adjectives ending in -e usually have the same singular form for both masculine and feminine.

So you get:

• un uomo intelligente
• una donna intelligente
• una decisione intelligente

Only the plural changes:

• uomini intelligenti
• donne intelligenti

Yes, but it is not exactly the same.

• È stata una decisione intelligente presents it as a completed past evaluation, and it is the most normal everyday choice.
• Era una decisione intelligente sounds more descriptive or like background information, depending on context.

So both are possible, but è stata is usually the better match for a simple completed past statement.

Yes, grammatically you can. Fu is the passato remoto of essere.

However:

• in everyday spoken Italian, è stata is much more common
• fu sounds more literary, historical, or regional

So for most learners, È stata una decisione intelligente is the most useful form to remember.

Not if you want the same idea as this sentence.

È stata intelligente usually means she/it was intelligent/smart, with intelligente describing the subject directly.

But È stata una decisione intelligente specifically says that something was an intelligent decision. The noun decisione is important, because it makes the idea more precise.