ANSWER is option 1, i.e. have spilled. The sentence uses the incorrect auxiliary verb 'has' for the main verb 'spilled'. Whenever the verb 'To have' is used as an auxiliary verb after a modal verb such as 'must' or 'should', it is always taken as 'have', for all nouns, singular or plural. E.g. She must have failed and They must have failed both will TAKE 'have'. Here, the sentence uses the modal verb 'must', showing insistence or obligation. This means even for the singular noun 'RANJAN', the correct auxiliary FORM would be 'must have spilled'.Thus, the rectified sentence is: Ranjan must have spilled the milk.