The correct option is
B heard
In order to replace the underlined part of the sentence with the correct form of the verb, we must understand the tense of the sentence.NOTE: The given sentence has two clauses. The first clause is in past perfect tense 'had called'. This explains to us that the action of calling was done before the action in the second clause. Hence, we can conclude that the tense of the second clause should be simple past tense for the sequences of actions to be in place.
Option B - 'Hear' is the original form of the verb and is used in the present tense. Hence, option B is incorrect.
Option C - 'Have heard' is the present perfect form of the verb. Though 'heard' is the past form of the verb 'hear', the helping verb 'have' makes it a part of present tense. Hence, option C is incorrect.
Option D - 'Was hearing' is the past continuous tense of the verb 'hear'. Hence, option D is incorrect.
Option A - 'Heard' is the past form of the verb 'hear' and it explains to us that 'After you called, they heard you'. Hence, option A is the correct answer.
'When you had called them, they heard you for the first time.'