Option A is incorrect. 'also', although suggests that Peter found a job along with Helen, it does not contain a main verb and is wrong.
Option B is incorrect. 'too' according to the rules of usage, must be used after the main verb and subject, unlike in this case. Thus, it is grammatically incorrect.
Option C is correct. 'so did' has a main verb and also agrees with the rules of usage. It also suggests that Peter found a job at the same time as Helen
Option D is incorrect. 'so' is incorrect as it does not contain a main verb. Further, 'so' is a conjunction and thus, it cannot be used immediately after 'and' independently. Thus, it is grammatically incorrect as well.