As three quantities are given, let us apply the formula

M₁H₁D₁=M₂H₂D₂

As per the question,

M₁=12 H₁=6 D₁= 15

M₂=? H₂=9 D₂=12

Apply the values to the formula,

$$\begin{gathered} 12\times 6\times 15=\mathrm{x}\times 9\times 12 \\ \mathrm{x}=\frac{12\times 6\times 15}{9\times 12} \\ \mathrm{x}=2\times 5 \\ \mathrm{x}=10 \mathrm{pumps} \end{gathered}$$

Hence option C is correct.