In one minute, 9 taps fill $\frac{1}{20}$^th part of water tank

similarly, in one minute, 1 tap fills $\frac{1}{20\times 9}=\frac{1}{180}$^th part of water tank

Let the number of taps used to fill the tnk in 15 min be x.

So, $\mathrm{x} \times \frac{1}{180}=\frac{1}{15}$

upon cross multiplication,

$\mathrm{x}=\frac{180}{15}=12 \mathrm{taps}$

Therefore, 12 taps can fill the tank in 15 minutes

Hence option C is correct.