The correct option is B Both assertion and reason are correct but reason is not a correct explanation of assertion.
further putting t=0 gives y=0. This means wave motion starts from mean position towards negative y−axis.
⇒ Assertion is correct.
The phase value (kx−ωt) signifies that the wave is moving towards +ve, x−axis.
⇒ Reason is correct.
But, reason is not correct explanation of assertion.
∴ Option (b) is correct.