The correct option is A Turn, will sound
A sentence that begins with 'if' is a conditional. These sentences refer
to hypothetical situations, 'what could happen' and 'what we wish would
have happened'.
The given sentence is also a conditional. It's a
type 1 conditional, which means that the if-clause is a possible
condition (turn the volume down) and the main clause is its probable result (the music will sound clearer). In this type, the tenses used are:
if-clause (simple present) + main clause (simple future)
Option A: The verb 'turn' is in the simple present, and 'will sound' is in
simple future. It adheres to the requirement for type 1 conditional.
Hence A is the correct option.
Option B: 'Could' and 'would' both are modals. They do not express the tense of the sentence.
Option
C: The tenses 'turn' and 'is sounding' refer to the simple present and present continuous, but that isn't correct. The main clause refers to a future
action, as it's the result and hasn't happened yet.
Option D: 'Would' and 'could' are modals and do not express the tense of the sentence.
Hence options B, C and D are incorrect.