Horned animals have different sizes of horns. The size of their horns can affect their ability to compete for a mate or provide protection from predators,
What is this an example of?
overproduction
O variation
survival of fit individuals

survival of fit individuals

Explanation:

I believe its survival of fit individuals because the animals with different horns are going to be able to find a mate or provide better depending on their horn and its kind of like natural selection in the sense those who have genetically better (??) horns are more likely to survive.  Hope it helps but it might be a little bit of a confusing explanation.