The correct option is (c) was.
"Each of the boys was rewarded." — 'Each' is a singular indefinite pronoun and always takes a singular verb.
When a sentence begins with 'each of', the subject is treated as singular (referring to one at a time), so a singular past tense verb 'was' is correct. Options (a) 'are being' and (b) 'have been' are plural/present forms; (d) 'were' is plural past — all incorrect here.