To reflect the function \( f(x) = x^2 - 1 \) over the y-axis, we replace \( x \) with \( -x \).
So, we calculate:
\[ g(x) = f(-x) = (-x)^2 - 1 \]
Since \( (-x)^2 = x^2 \), we have:
\[ g(x) = x^2 - 1 \]
Thus, the equation that correctly identifies \( g(x) \) is:
D. \( g(x) = x^2 - 1 \)