To evaluate the expression \( \sqrt{19} - 32 \):
- \( \sqrt{19} \) is an irrational number because it cannot be expressed as a fraction of two integers.
- Subtracting 32 (which is a rational number) from an irrational number results in an irrational number.
Thus, the result of the expression \( \sqrt{19} - 32 \) is an irrational number.
The correct response is:
C. an irrational number.