A sentence is in active voice when the subject performs an action.
The given sentence is in active voice: subject (they) performs an action (have built) against the object (a new hospital).
Its voice will hence be changed to passive voice.
When a sentence is in passive voice, the subject will receive the action.
While changing the voice, the meaning should not change. Hence the word order must be altered, in a way that the subject receives the action. So, the subject is replaced by the object and vice versa.
Also, the tense ('have built' is in present perfect tense) must remain the same, to keep the meaning intact.
Option C: The sentence is in passive voice.
The subject 'a new hospital' receives the action 'has been built'. It is alright to omit the object 'by them' since the meaning is implied. Also, the verb form 'has been built' remains the same 'present perfect tense'.
Hence option C is correct.
Option A: 'Is built' is in simple present tense and is different from the original tense of the sentence.
Option B: 'Was built' is in simple past tense.
Option D: 'Had been built' is in past perfect tense.
These verb forms do not express the same tense as well as overall meaning of the original sentence.
Hence options A, B and D are wrong.