Final answer:
The number of castrated males, also known as wethers, on the farm is 20. So, the correct answer is option a.
Step-by-step explanation:
The question asks us to calculate the number of castrated male goats on a farm. The term for a castrated male goat is a wether. Given that the farm has 60 does (female goats), 30 bucks (intact male goats), 40 kids (young goats), and 20 wethers (castrated male goats), the answer to the question "How many castrated males are on the farm?" is simply the number of wethers.
Therefore, the correct answer is 20 wethers.