Which parasite is described as being deposited by flies around the nostrils?

Nasal bots are the correct answer because these are the larvae of a specific type of fly known as the botfly. The adult females deposit their eggs around the nostrils of animals, primarily cattle, and sometimes sheep or horses. Once the eggs hatch, the larvae enter the nasal passages and sinuses of the host, leading to various respiratory issues. This life cycle is directly connected to the behavior of the flies, which target the mucous membranes around the nostrils for egg-laying.

The other options refer to different types of parasites that do not have the same reproductive and developmental characteristics as nasal bots. Heel flies, for instance, are also flies that affect livestock, but they primarily target the skin, leading to the development of larvae in the animal's back rather than around the nostrils. Ticks and fleas are external parasites that feed on blood and do not have any direct relationship with the nasal area for depositing larvae. This distinction reinforces why nasal bots are the specific answer related to deposition by flies around the nostrils.