The given sentence states a fact: something that takes place every four years. Hence the most appropriate verb form will be in the present tense.
Option C: 'Are held' expresses the simple present tense. This verb form states actions that are daily habits or facts. It applies to the sentence here, that states a fact: 'elections for President are held every four years'. Hence option C is correct.
Option A: 'Held' is the main verb and does not have any helping verb. An auxiliary/helping verb is essential to express a tense.
Option B: Similarly, 'hold' without a helping verb does not express any tense.
Option D: 'Were held' refers to a past activity, whereas the sentence states a present fact.
Hence options A, B and D are incorrect.