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

$$\begin{gathered} \frac{1}{\mathrm{x}}=\frac{1}{9} \\ \therefore \mathrm{x}=9 \end{gathered}$$

The pipe will take 9 hours to fill the tank.

 Hence option A is correct