The correct option is
C Is sitting
Option A - 'Sitting' is the continuous form of the verb 'sit' which needs an auxiliary to determine the tense of the sentence. Without an auxiliary, the sentence is never complete. Thus option A is incorrect.
Option B - 'Has been sitting' is used to describe a sentence in the present perfect continuous tense. Hence, option B is incorrect.
Option D - 'Has been sit' is grammatically incorrect as to form the sentence in the present continuous tense, we need an auxiliary and the verb in the participle form. Hence, option D is incorrect.
Option C - 'Is sitting' is grammatically correct as it has the auxiliary 'is' which denotes present tense and 'sitting' is the continuous form of the verb which denotes continuous tense. 'Is sitting' is used to define a sentence in the present continuous tense. Thus option C is the correct answer.
'She is sitting with Barbie.'