The correct option is B hear
The verbs of sense (see, hear, taste, feel, smell) are generally used in the simple present tense form. The underlined phrase 'am hearing' sounds unnatural in this sentence. Hence it must be changed.
Option B: 'Hear' is a verb in the simple present tense, it agrees to the sentence as the sentence itself is in the present tense. Hence B is the correct option.
Option A: 'Heard' is in the simple past whereas the sentence is in the present tense.
Options C and D: Similarly, 'was hearing' is in the past continuous tense and 'have been hearing' is in the present perfect continuous tense. As mentioned earlier, the verbs of sense are not used in the continuous form. The exclamation 'Hush!' also suggests that the action 'I hear' is in the immediate present.
Hence options A, C and D are incorrect.