We are given:
\[
I = \int_{-1}^1 \sin x \cos^3 x \, dx
\]
The integrand is an odd function because \( \sin x \) is odd and \( \cos^3 x \) is even. The product of an odd function and an even function is odd, and the integral of an odd function over a symmetric interval \( [-1, 1] \) is zero.
Thus, the correct answer is \( 0 \).