The obnoxious visitors that arrive at the hill station are leopards and leeches. Let's discuss each option to understand why option 'A' is the correct answer.

a) Leopards and leeches:
- Leopards are large, carnivorous cats found in many parts of the world, including hill stations. They can be dangerous to humans and other animals.
- Leeches are blood-sucking parasites that live in moist environments such as forests and hill stations. They attach themselves to animals, including humans, and feed on their blood. Leech bites can be irritating and cause discomfort.

b) Drongos and minivets:
- Drongos are small passerine birds known for their aggressive behavior. They are not typically considered obnoxious visitors as they do not pose a threat to humans.
- Minivets are colorful birds found in hill stations and other forested areas. They are not known to be obnoxious or harmful.

c) Drongos and leopards:
- Drongos are small birds, as mentioned before, and are not obnoxious visitors.
- Leopards are indeed obnoxious visitors as they can be dangerous to humans and other animals. However, this option also includes drongos, which are not considered obnoxious.

d) Leopards and minivets:
- Minivets are colorful birds, as mentioned before, and are not obnoxious visitors.
- Leopards are obnoxious visitors as they can be dangerous. However, this option also includes minivets, which are not considered obnoxious.

Therefore, the correct answer is option 'A' - leopards and leeches - as both leopards and leeches can be obnoxious visitors to the hill station. Leopards pose a threat to humans and other animals, while leeches can cause discomfort and irritation with their blood-sucking behavior.

A