To find \(\sin^2\) if \(A + B = 180^\circ\), use the following trigonometric identity:

1. Identity:
\[
\sin^2 A + \cos^2 A = 1
\]
For angles that add up to \(180^\circ\), we use:
\[
\sin (180^\circ - \theta) = \sin \theta
\]
Therefore,
\[
\sin^2 A = \sin^2 (180^\circ - B)
\]
This simplifies to:
\[
\sin^2 A = \sin^2 B
\]

2. Calculate:
Since \(A + B = 180^\circ\), \(\sin^2 A = \sin^2 (180^\circ - B) = \sin^2 B\).

Therefore:
\[
\sin^2 A = \sin^2 B
\]
\[
\sin^2 A + \sin^2 B = 1
\]

Since \(\sin^2 A + \sin^2 B = 1\), \(\sin^2 A\) will be 0 if \(A\) or \(B\) is \(0^\circ\) or \(180^\circ\).

Thus, the correct answer is:

A. zero