An action in a sentence can be represented in two ways, namely active and passive voice.
The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in the active voice.
The correct answer is C)How are languages learnt?
The option C is correct as the given question is in the active voice and to transform it into the passive voice, the object of the given question(languages) comes in the place of the object(people) and vice versa.'People' should be omitted as there isn't any specific person or group mentioned. Also, as the sentence is in the simple present tense, the past participle form of the main verb (learn) is used.
Options A, B, and D are wrong as the sentence is changed, 'do' cannot be added, and 'were' cannot be used respectively.