We should not allow mosquitoes to breed in our surroundings because they

Are vectors for many diseases

The correct answer is Option (2) → Are vectors for many diseases 

Mosquitoes are vectors, which means they transmit disease-causing organisms from one person or animal to another. They are not harmful just because of their bites, but because they can carry and spread dangerous viruses and parasites.

Some of the serious diseases spread by mosquitoes include:

• Malaria – Caused by a parasite, transmitted by Anopheles mosquitoes.
• Dengue – A viral infection spread by Aedes mosquitoes.

These diseases can lead to illness, long-term health problems, and even death. That's why it's very important to stop mosquitoes from breeding, especially in stagnant water, where they lay their eggs.

Other options :

• "Multiply very fast and cause pollution" – While mosquitoes breed quickly, they do not cause pollution. Pollution is caused by chemicals, plastics, and waste, not insects.
• "Bite and cause skin diseases" – Mosquito bites can cause itching and minor irritation, but they do not typically cause skin diseases. The main danger is the diseases they carry, not the bites themselves.
• "Are not important insects" – This is not true. All species have a role in the ecosystem. Some mosquitoes are food for birds, fish, and frogs. However, the health risks they pose as disease carriers are the reason we must control their population.