To determine how long it will take to fill 72 gallons of petrol, given that it takes 15 minutes to fill 125 gallons, follow these steps:

1. Calculate the rate of filling in gallons per minute:

\[
\text{Rate} = \frac{125 \text{ gallons}}{15 \text{ minutes}} = \frac{25}{3} \text{ gallons per minute}
\]

2. Use this rate to determine the time required to fill 72 gallons:

\[
\text{Time} = \frac{72 \text{ gallons}}{\frac{25}{3} \text{ gallons per minute}} = 72 \times \frac{3}{25} = \frac{216}{25} = 8.64 \text{ minutes}
\]

So, the correct answer is:

C. 87 minutes