Correct Answer:
C. spilt
The correct answer is spilt. The idiom is 'cry over spilt milk', which means to be upset about something that has already happened and cannot be changed. The correct form is the past participle 'spilt' (or 'spilled' in American English). 'Boil' and 'cry' do not form this idiomatic expression, and 'None' is incorrect. Therefore, 'spilt' is the correct word to complete the idiom.