Evaluate the expression, or state that the expression is not a real number.

To evaluate the expression \( \sqrt{100 - 36} \):

1. First, calculate \( 100 - 36 \):
\[
100 - 36 = 64
\]

2. Next, find the square root of 64:
\[
\sqrt{64} = 8
\]

Since both calculations result in a valid real number, we conclude that the expression is a real number.

The correct choice is:

A. The expression is a real number. \( 100 - 36 = 64 \)