Which of the following is an external parasite that affects cattle?

Cattle are commonly affected by various external parasites, among which ticks are significant. Ticks attach to the skin of cattle, where they feed on their blood. This feeding not only causes discomfort for the animals but can also lead to other health issues, such as the transmission of diseases like anaplasmosis and bovine babesiosis. Ticks are often found in pastures and wooded areas and can thrive in various environmental conditions, making them a persistent issue for cattle farmers.

Worms, while also parasites, are internal parasites and not external, as they live within the body of the host. Fleas are an external problem typically associated with pets, like dogs and cats, rather than cattle. Protozoa are another type of internal parasite that generally live inside the host and cause diseases but are not classified as external parasites affecting cattle. Therefore, the correct answer highlights the impact of ticks on cattle as external parasites.