The correct option is
D We gave our colleague a gift when she retired.
The given sentence is in the passive voice, as the subject 'Our colleague' indirectly receives an action 'was given' (and directly receives the direct object 'a gift').
If this sentence is changed to active voice, then the subject is also changed, in order to retain the meaning of the sentence.
'Subject (Our colleague) + verb (was given) + direct object (a gift) + adverb clause (when she retired)' changes to:
'New subject (We) + verb (gave) + indirect object (our colleague) + new object (a gift) + adverb clause (when she retired)'
The active form of 'was given' in the simple past tense is 'gave'.
This can be seen in option D, hence it's the right answer.
(The new subject 'we' is implied in the passive voice: Our colleague was given a gift by us when she retired.)
Option
A: The sentence has changed in meaning, as the verb form alone has been changed from passive to active.
Option B: This sentence is the same as the original one. The only change made is that the implied words 'by us' are explicit in this sentence.
Option C: This sentence is still in passive voice 'was given' is the passive verb form.
Hence options A, B and C are incorrect.