The sentence indicates that the subject cannot hear what the object is saying and so, he/she is asking the object to decrease the volume of the radio. Thus, the answer must mean 'decrease the volume'.
Option A is incorrect.'turn up' means 'to increase the volume' and not 'decrease the volume'. Thus, it does not have the required meaning and is not the correct answer.
Option B is incorrect. 'turn on' means 'to switch on' and not 'decrease the volume'. Thus, it does not have the required meaning and is not the correct answer.
Option C is incorrect.'turn away' means 'to refuse or reject' and not 'decrease the volume'. Thus, it does not have the required meaning and is not the correct answer.
Option D is correct.'turn down', in this context, means 'to decrease the volume'. Thus, it has the required meaning and is the correct answer.