The correct option is
A A beautiful flower pot was given to me by him.
A sentence is in the active voice when the subject performs an action.
A sentence is said to be in the passive voice when the subject receives an action.
The given sentence is in the active voice.
If this sentence is changed to passive voice, then the subject is also changed, in order to retain the meaning of the sentence.
So,
'Subject (He) + verb (gave) + indirect object (me) + direct object (a beautiful flower pot)' changes to:
'New subject (A beautiful flower pot) + verb (was given) + indirect object (to me) + by + new object (him)'
The
passive form of 'gave' in the simple past tense is 'was/were given'.
This can be seen in option A, hence it's the right answer.
Option B: The verb form 'had given to me' is in active form. As a result, the meaning of the sentence is incomplete, as the rest of the sentence is in passive voice.
Option
D: Similarly, in this option, the verb form 'had been giving' is in the active form. It expresses the past perfect continuous tense, and does not express the simple past tense of the given sentence.
Option C: This sentence is in the active voice, expressing the past progressive tense. It has changed in meaning.
Hence options B, C and D are incorrect.