As the given passage is in the past tense, we must fill in the blank using the past tense of the verb 'hear'.
'Hardly had I entered the house when I _____ the mirror break.' The 'had' explains that the action of entering the house took place before the breaking of the mirror. Hence, the first half of the sentence is in the past perfect tense and the second half of the sentence should be in the simple past tense.
Option A - 'Hear' is the original form of the verb and can't be used in past tense. Thus option A is incorrect.
Option C - 'Heared' is an incorrect word. Thus option C is incorrect.
Option D - 'Had heard' is the past perfect form and can't be used in simple past tense. Thus option D is incorrect.
Option B - 'Heard' is the past form of the verb 'hear' and can be used to frame a sentence in the simple past tense. Thus option B is the correct answer.
'Hardly had I entered the house when I heard the mirror break.'