According to the formula, if a pipe empties $\frac{1}{\mathrm{x}}$ part of tank in 1 hour, then it takes x hours to fully empty the tank.

$$\begin{gathered} \frac{1}{\mathrm{x}}=\frac{1}{7} \\ \therefore \mathrm{x}=7 \end{gathered}$$

The pipe will take 7 hours to empty the tank.

Hence option C is correct