The correct option is C Shrunk
'Shrink' is the base form of the verb, since it is an irregular verb its past form is 'shrank' and its past participle form is 'shrunk'.
Therefore, option A 'shrink' and option B 'shrank' are incorrect whereas, shrinking is the continuous form of the verb 'shrink', therefore option D is incorrect.
The correct option is option C 'shrunk'.