Question

Read the sentence.

Answer 1

The correct answer is B.

Scientists reached a decision about when to begin the experiment.

Transitive verbs have both active and passive forms.

Active : The man killed the wolf.  The subjects acts upon the verb in this sentence.

Passive : The wolf was killed by the man. The subject is being acted upon by somebody or something.

The passive forms are made up of the verb be with the past particle.

A decision was reached by scientist is used in the passive voice. A decision is being acted upon by the scientists.

Scientists reached a decision is in an active voice. Scientists ( the subjects ) act upon the verb.

Answer 2

The revision of the sentence that makes it use the active voice is Scientists reached a decision about when to begin the experiment.
The other sentences are still in the passive voice.