Final answer:
The form of the underlined verb 'was given' in the sentence is the past passive, with 'was' as the past tense of the auxiliary verb 'to be' and 'given' as the past participle of 'to give'.
Step-by-step explanation:
In the sentence, "I was given a necklace," the form of the underlined verb was given is the past passive. This verb form is used when the subject of the sentence is the recipient of the action rather than the one performing it. The verb "was" is the past tense form of the auxiliary verb "to be", and "given" is the past participle of the verb "to give".