Final answer:
Eosinophils are the type of leukocytes that specifically target and defend against helminthic (parasitic worm) infections by secreting toxic substances. The correct option is A.
Step-by-step explanation:
The type of leukocyte that defends against helminthic infections is the eosinophil. These white blood cells specialize in combating parasitic infections, such as those caused by worms, and they are particularly effective due to the release of a range of highly toxic substances.
Eosinophils stand out for their granules and distinctive bilobed nucleus, and their role also extends to involvement in allergic reactions, where they, along with basophils, release histamine and other proinflammatory compounds.
Therefore, the correct option is A.