The correct option is C is awarded
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 C: The given sentence states a custom by "our school". Hence it's in simple present tense. So, the simple present form of 'to be', 'is' is used. (The simple present tense states a custom or habit.)
'Is awarded' is the appropriate structure, formed by the simple present tense verb 'is' + past participle (awarded). Hence option C is correct.
Option A: 'Is award' is grammatically incorrect. 'Award' is the base verb and must be modified in this sentence to express the tense of the sentence. Hence A is incorrect.
(Base verbs like 'award' are used in the first person point of view (I, we) and the future tense.)
Option B: 'Awards' is a verb in the active form: A gold medal awards to the topper... It changes the thing "gold medal" to perform, rather than receive, the action. Hence B is incorrect.
Option D: 'Is awarding' is not the correct verb form used in the passive voice. 'Awarding' is a present participle whereas the required verb form is a past participle. Hence D is incorrect.