Which variable is discrete
a time to finish a race
b temperature at start of race
c number of runner in race
d length of race in kilometers

C. The number of runners in a race

Step-by-step explanation:

Variables can be widely be classified as either discrete or continuous. A continuous variable is a variable which can take on any value within its domain. A continuous variable can take on fraction of units. Examples of continuous variables include;

time to finish a race - it can be 10.5 sec or 35.56 sec

temperature at start of race - the temperature could be 25.5 K, or 45.57 K

length of race in kilometers - the length could be 20 km, 22.45 km or 38.8 km

All the above variables can be measured in fraction of units and are thus continuous in nature.

On the other hand, a variable is said to be discrete if it can take on only integer values. An example in this case would be ;

the number of runners in a race. We can only have, 10, 45, 600, 565 and so on runners but never a fraction of a runner.