The present perfect tense is used to indicate an action which started in the past and is continuing now. It is often used along with 'since or 'for' to indicate the time. It is made of two elements: (1) the appropriate form of the auxiliary verb, 'to have' (in present tense) (2) the past participle of the main verb.
In the given sentence the words, 'since 1994' is used. This means that Ramesh was a teacher in the past since 1994 and is still continuing to be one. Hence its present perfect form is to be used. As Ramesh is a singular noun (third person), the appropriate form of the auxiliary verb, 'to have' is: has. The main verb is 'be', whose past participle form is 'been'. Hence, 'has been' as given in option C is correct.
Option A is incorrect as it is in simple present tense.
Option C is incorrect as it is in present continuous tense.
Option D is incorrect as it is simple past tense.