The main function of white blood cells is to combat infections. 

Explanation: White blood cells are part of the immune system, which is responsible for defending the body against invading pathogens like bacteria, viruses, and parasites. They identify and attack these invaders, helping to prevent and fight infections. 

Why other options are incorrect:

• To carry nutrients: The primary function of red blood cells is to carry nutrients. Red blood cells transport oxygen and carbon dioxide. While white blood cells play a role in maintaining overall health, they are not the primary transporters of nutrients. 
• To carry oxygen: As mentioned above, this is the function of red blood cells. Red blood cells contain hemoglobin, a protein that binds to oxygen and delivers it to tissues. 
• To give strength: White blood cells are important for fighting infection, but they do not directly contribute to physical strength. Muscle tissue and other factors are responsible for determining strength.