Варіантні форми: коли норма допускає дві

Two correct answers

At lower levels a form is right or wrong. At C1 you meet the places where the norm admits both, and the question changes from which is correct to which fits here. That is a stylistic judgement, and it is learnable.

Instrumental singular feminine: -ой and -ою

вадой / вадою, рукой / рукою. Both are standard. The longer form is bookish and belongs to poetry, song and elevated prose; the shorter is the everyday one. In folk song the longer form is the norm, partly because it fits the metre.

Genitive singular masculine: -а and -у

This one is not free - it is largely semantic, and the split is worth learning:

Some words take both, with different meanings: з дома - out of the building, з дому - from home.

Short and long prepositions

ля and каля, з and са, у and ва, аб and аба. The longer forms appear before difficult consonant clusters - са мной, ва ўсім - and the choice is phonetic, not stylistic. Ля against каля, however, is free.

Зараз and цяпер

Not variants at all, but constantly confused, so it belongs here: цяпер is now, the present period. Зараз is in a moment, immediately ahead. Цяпер я жыву ў Мінску - these days. Зараз прыйду - I am coming right now. Using зараз for the general present is one of the commonest interference errors.

Participle or clause

Where Russian would use a present active participle, Belarusian normally prefers a clause with які. Both may be grammatical, but the clause is the natural choice and the participle often reads as translated. This is a variant question where one option is markedly better.

How to choose

Three questions: Is the difference semantic (then it is not a variant at all)? Is it phonetic (then the context decides)? Is it stylistic (then match the register you are writing in, and be consistent within one text)?