We are asked to integrate \( e^{3x} \). The integral of \( e^{ax} \) is:
\[
\int e^{ax} \, dx = \frac{e^{ax}}{a} + k.
\]
In our case, \( a = 3 \), so:
\[
\int e^{3x} \, dx = \frac{e^{3x}}{3} + k.
\]
Thus, the correct answer is option (C) \( e^{3x} + k \).