The given sentence states a habit or a general truth: 'every evening' presents a daily habit.
The simple present tense presents a truth or habit. Hence, the sentence is in simple present tense.
Also, it's in passive voice, because the subject 'the park gates' experiences the action 'to be locked'.
Option B: 'Are locked' is in the simple present tense, in passive voice. It presents a daily habit 'are locked at 6:30 p.m. every day'. Hence option B is correct.
Option A: 'Locks' is a simple present tense verb in active voice. The sentence is in active voice when the subject 'the park gates' performs an action 'to lock'. But here it undergoes the action 'to be locked'.
Option C: 'Locked' is a simple past tense verb whereas the sentence is in present tense.
Option D: 'Were locked' is also in past tense in passive voice, as opposed to the sentence.
Hence options A, C and D are wrong.