Given that:
4 friends: A, B, C and D.
From statement I: C is between A and D.
Either A or D is sitting to the immediate right of C.
Hence, the data in statement I alone is not sufficient to answer the question.
From statement II: D is to the immediate right of C.
Either A or B is sitting to the immediate left of C.
Hence, the data in statement I alone is not sufficient to answer the question.
By combining I and II
We can conclude that A is to the immediate right of B.
Hence, the data in both statements I and II together are sufficient to answer the question.