Question

Venus can often be seen IN THE NIGHT SKY. A.adjective B. adverb C.appositive

Answer 1

The correct answer is B, adverb. Where can Venus often be seen? - In the night sky. Because you are looking for a location, it is an adverb, not an adjective.
And appositive would be, for example, 'Venus, which can often be seen in the night sky, is beautiful', where 'which can often be seen in the night sky' is appositive. 

Answer 2

'In the night sky' is an appositive.