The correct option is B the dirtiest
The Superlative Degree denotes the existence of the highest degree of the quality. It is used when more than two things are compared.
The sentence in the question is in superlative degree, thus the adjective used must be in superlative form.
Option C: 'The dirtiest' is in superlative form so it is correct.
Option A and Option B: 'dirtier' and 'the dirtier' are in comparative form so both options are incorrect.
Option D: 'the dirty' is in positive form so it is incorrect.