'It's' is in simple present tense. The simple present tense states a truth or a fact. The next sentence will also be in the same tense, because it too states a fact.
Option C: 'Are employed' is in the simple present tense, in passive voice. It presents the state of 'five hundred people' there. Hence option C is correct.
Option A: 'Are' is a simple present tense verb in active voice. The sentence is in active voice when the subject 'five hundred people' performs an action 'to employ'. But here it experiences the action 'to be employed'.
Option B: 'Employed' is a simple past tense verb whereas the sentence is in present tense.
Option D: 'Were employed' is also in past tense in passive voice, as opposed to the sentence.
Hence options A, B and D are wrong.