Leprosy, also known as Hansen`s disease, is caused by bacteria. This bacteria is called Mycobacterium leprae.

Option 1, virus, is not the correct answer. Leprosy is not caused by a virus. Viruses are different from bacteria and do not cause the same type of infections.

Option 3, protozoa, is also not the correct answer. Protozoa are single-celled organisms, different from bacteria. Protozoa can cause diseases like malaria and amoebic dysentery, but they do not cause leprosy.

Option 4, retrovirus, is also not correct. Retroviruses are a specific type of virus that have RNA as their genetic material. While retroviruses can cause diseases like HIV, they do not cause leprosy.

Therefore, option 2, bacteria, is the correct answer. Leprosy is caused by the bacteria Mycobacterium leprae, which affects the skin, peripheral nerves, mucous membranes, and eyes. It is a slow-growing bacteria and has a long incubation period, which means symptoms may take years to develop.