Active Voice is usually preferred.
Passive Voice - the subject receives the action and it might have the phrase “by the…”. Passive Voice is usually avoided except in 2 cases:
- Subject of the sentence is indefinite/general.
E.g. Rice is not grown in these fields (good). They do not grow rice in these fields (poor) - Depends on what the writer emphasises.
E.g. America was discovered by Columbus. or Columbus discovered America.
Some Links:
gmat