Present perfect tense is used to indicate an action which is not defined by a time of occurrence. Here, the context will take the form S + has + V3 + O.
"Tell" in option A is incorrect as this is the base form of the verb and is not used in present perfect tense.
"Tells" in option B is incorrect as the context refers to an action which is not defined by time of occurrence. Thus, present perfect tense will be used here. 
"Has told" in option C is correct as this is the correct present perfect perfect tense form of the verb. It is used with the auxiliary verb 'has/have' and the past participle form of the verb.
"Would tell" in option D is incorrect as this indicates the conditional present, whereas the time of occurrence is not mentioned here. So, present perfect should be used.
Thus, option C is correct.