Final answer:
The correct answer is eosinophils, which are white blood cells that increase in response to allergic reactions and parasitic infestations.
Step-by-step explanation:
The type of white blood cells that are usually dark red and increase in numbers in patients with allergic reactions or in parasitic infestations are eosinophils. Eosinophils are part of the group known as granulocytes, which also include neutrophils and basophils. While neutrophils are the most numerous and primarily fight bacterial infections, eosinophils are specialized for targeting parasitic infections and play a significant role in the body's response to allergens by releasing histamine and other proinflammatory compounds from their granules upon stimulation. Therefore, the correct answer to the question is option 4) Eosinophils.