The correct passive form of the given sentence is:

☀ A watch had been gifted to him by us on his birthday.

Explanation:

In the active sentence, the subject is "We," and the verb is "had gifted" (past perfect tense). The object is "him," and the direct object is "a watch."

To convert this to the passive form, we make the direct object ("a watch") the subject of the passive sentence and use the appropriate form of the verb "to be" (in this case, "had been") followed by the past participle of the main verb "gift" ("gifted"). Additionally, we retain the agent "by us" to indicate who performed the action in the passive sentence.

Let's break down the options:

1. A watch have been gifted to him by us on his birthday. (Incorrect) This option uses the incorrect verb form "have been gifted." The original sentence is in the past perfect tense "had gifted," so the correct form should be "had been gifted."

2. A watch had been gifted to him by us on his birthday. (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence. It uses the correct form "had been gifted."

3. A watch had been gifted to him by we on his birthday. (Incorrect) This option uses the incorrect pronoun "we" instead of "us." The correct form should use "us" to indicate the correct object pronoun.

4. A watch was gifted to him by us on his birthday. (Incorrect) This option changes the meaning of the original sentence by using the simple past tense "was gifted" instead of the past perfect tense "had been gifted." The past perfect tense in the original sentence indicates that the gifting occurred before some other past event, while the simple past tense suggests a single event in the past without specifying any temporal relationship with other events. The correct passive form should use the past perfect tense "had been gifted," as shown in option 2.