Select the option that expresses the given sentence in active voice.
Pudding had already been prepared by her.

Correct option is B

Option (b) is the correct active voice of the given sentence.
Active Voice: In Active voice the subject is the one doing the action.
Passive Voice: In Passive voice the subject is the recipient of the action.
To convert active voice into passive voice the object becomes the subject and the subject becomes the object.
In Past Perfect Tense as given in the question the structure for changing a sentence from active to passive and vice versa is:
Subject + had + v3+ object. (Active voice)
Object+ had+been +V3+ by + subject. (Passive voice)
The active voice of the sentence is: "She had already prepared pudding."