Explanation

From sin^2 x + sin x = 1 we get sin x = 1 - sin^2 x = cos^2 x. Substituting cos^2 x = sin x into the expression gives sin^6 x + 3 sin^5 x + 3 sin^4 x + sin^3 x. This is the expansion of (sin^2 x + sin x)^3, and since sin^2 x + sin x = 1, its value is 1^3 = 1. Hence option C.