Correct Answer: Depression

Solution : Given:
Depression, Malaria, Typhoid, Flu (The given words are related to diseases such as physical disease and mental disease.)

Let's check each option –
First option: Depression; It is a constant feeling of sadness. It is related to mental illness and it is a mental disease.
Second option: Malaria; It is an infection caused by a plasmodium parasite transmitted by the bite of infected mosquitoes. It is a physical disease.
Third option: Typhoid; It is a bacterial infection that spreads through contaminated food and water. It is a physical disease.
Fourth option: Flu; It is a common viral infection that can be deadly, especially in high-risk groups. It is a physical disease.

Therefore, depression is the odd one out as depression is a mental disease whereas all others are physical diseases that occur in the body. Hence, the first option is correct.