The correct option is
B the party was
The word 'asked' in the sentence indicates that the event occurred in the past. Thus, the answer must be in the past tense.
Option A is incorrect. 'is the party' is in present tense. The tense of the sentence is simple past and so, using a phrase in present tense will make the tense of the sentence inconsistent.
Option B is correct. 'the party was' is in simple past tense. Thus, it matches the tense of the sentence and is the correct answer.
Option C is incorrect. Although 'was the party' is in past tense, according to the rules of usage, the verb in reported speech must be at the end of the clause. Since 'was' is in the beginning of the clause, the phrase is grammatically incorrect.
Option D is incorrect. 'is party' is not only in the wrong tense but is also grammatically incorrect as it does not have a definite article before 'party'. Since the subject is asking about a specific party, there must be a definite article before it.