Option 1 states that both the assertion (A) and the reason (R) are true, and that R is the correct explanation of A. Let`s analyze both statements separately to validate this option.

Assertion (A) states that sterilized milk usually spoils upon long exposure to air. This means that if sterilized milk is left exposed to air for a long period of time, it will eventually spoil. This is generally true because even sterilized milk can still be contaminated with spoilage organisms or bacteria present in the surrounding air. These organisms can grow and multiply in the milk if given enough time.

Reason (R) states that air supplies necessary oxygen to spoilage organisms present in milk. Oxygen is indeed necessary for the growth of spoilage organisms, including bacteria, which can spoil the milk. So, R correctly explains why A is true.

Hence, option 1 is the correct answer, as both A and R are true and R is the correct explanation of A.