A sentence in passive voice focuses on the person or thing that receives an action rather than the person/thing that performs the action.
For example, 'My room was cleaned by me.'
The basic structure of passive voice is:
the appropriate form of the verb 'to be' + past participle
Option B: 'Were not picked up' is the appropriate structure, formed by the past tense verb 'were' + not + past participle (picked up). Hence option B is correct.
Option A: 'Was' is a singular verb whereas the subject (nuts) is plural. The subject and verb must agree with one another in terms of number. Hence A is incorrect.
Option C: 'Did' is a verb in the active voice. The word itself means 'to perform an action'. It changes the subject to perform, rather than receive, an action. Hence C is incorrect.
Option D: 'Pick up' is the base verb that does not express the past tense of the sentence. The past participle 'picked up' must be used. Hence D is incorrect.
Note: 'Pick up' is a phrasal verb, i.e., it's a phrase made with a verb 'pick' and a preposition 'up'.