The correct answer is option 2 - allergy. Hay fever, also known as allergic rhinitis, is a type of allergic reaction that occurs when your immune system overreacts to allergens in the air. Common allergens that can trigger hay fever include pollen, dust mites, pet dander, and mold spores.

Option 1 - malnutrition, is not related to hay fever. Malnutrition is a condition caused by a lack of proper nutrition or specific nutrients.

Option 3 - old age, is also not related to hay fever. There is no direct association between hay fever and aging.

Option 4 - overwork, is also not a sign of hay fever. Although stress and exhaustion may worsen symptoms for some individuals, overwork itself does not cause hay fever.

Therefore, option 2 - allergy, is the correct answer as hay fever is primarily caused by allergic reactions to certain substances.