Option 1 is not correct because although both the assertion and reason are true, the reason does not correctly explain the assertion.

Option 2 is incorrect because the reason is not the wrong explanation for the assertion; the reason is a correct statement but it doesn`t relate to the assertion in context.

Option 3 is untrue because although the assertion is indeed correct, the reason is also true and not false.

Option 4, the correct answer, is stating that the assertion is wrong but the reason is correct. The velocity of a jet aircraft at Mach 1 is the same whether at an altitude of 15 km or at sea level, making the assertion false. However, the velocity of sound does change according to the temperature of the surrounding medium, which makes the reason true.