A sentence is in active voice when the subject performs the action.
A sentence is in passive voice when the subject receives the action.
While converting a sentence from active to passive, the subject becomes the object and vice versa.
Subject + verb + object (active voice)
Subject (The sack) + verb phrase (is put) + by + object (her) (passive voice)
In the given sentence, 'puts' (simple present) changes its form to 'is put' (simple present, passive).
This can be seen in option C: The sack is put on the floor by her.
Hence C is correct.
Option A is incorrect because the tense of the sentence has changed to past 'was'.
Option B is incorrect because the verb has been changed to plural. As the subject 'sack' is singular, the verb must agree with the subject, and be singular.
Option D is incorrect for the same reasons, 'were' is in past tense and indicates that the subject is plural.