Final answer:
The powerful bronchoconstrictor released by mast cells is leukotrienes, significant in causing bronchoconstriction and associated with allergic reactions.
Step-by-step explanation:
The powerful bronchoconstrictor released by mast cells is d) Leukotrienes. These substances are involved in inflammatory and allergic reactions. When mast cells encounter an allergen, they can release a variety of substances, including histamines and leukotrienes. While histamines are also involved in allergic reactions, leukotrienes play a particularly significant role in bronchoconstriction, which is the narrowing of the airways. Leukotrienes are part of a group of newly formed lipid mediators which come from membrane arachidonic acid metabolism, and they contribute to the symptoms of asthma and other allergic reactions.