When a runner was crossing the 12 km mark, she was informed that she had completed only 80% of the race. How many kilometres was the runner supposed to run in this event?

The correct option is 15.

To find the total distance of the race, we can set up a simple algebraic equation based on the information provided in the question.

Let $x$ represent the total distance of the race in kilometers.

We are given that the runner has completed 80% of the total distance when she reaches the 12 km mark. We can write this relationship as:
$80\%\text{of}x=12$

To solve for $x$, we first convert the percentage into a decimal:
$0.80·x=12$

Next, isolate $x$ by dividing both sides of the equation by 0.80:
$x=\frac{12}{0.80}$

Calculating the division gives:
$x=15$

Therefore, the runner was supposed to run a total of 15 kilometers in this event.