Mosquitoes are vectors for
I. Dengue fever
II. Malaria
III. Cholera

Options

A. I only
B. I and II
C. II and III
D. I,II and III

Explanation

To determine which diseases mosquitoes are vectors for, we need to analyze each option provided in the question:

1. **Dengue fever**:
- Dengue fever is indeed transmitted by mosquitoes, specifically by the Aedes aegypti mosquito. Therefore, this statement is correct.

2. **Malaria**:
- Malaria is caused by the Plasmodium parasite, which is transmitted through the bites of infected Anopheles mosquitoes. Hence, this statement is also correct.

3. **Cholera**:
- Cholera is primarily caused by the bacterium Vibrio cholerae, which is spread through contaminated water and food. Mosquitoes are not involved in the transmission of cholera. Therefore, this statement is incorrect.

Based on this analysis, we conclude that mosquitoes are vectors for **Dengue fever** and **Malaria**, but not for **Cholera**.

### Final Answer:
Mosquitoes are vectors for:
- I. Dengue fever: Correct
- II. Malaria: Correct
- III. Cholera: Incorrect

Thus, the correct options are I and II.