Why can dinoflagellate blooms be harmful to humans?
a. dinoflagellates produce toxins that kill shellfishb. during algal blooms, dinoflagellate population densities reach very high levelsc. dinoflagellate blooms are not harmful; they are just unsightlyd. people can get paralytic shellfish poisoning during dinoflagellate blooms

Dinoflagellate is a type of neurotoxin which is found in the red-tide algae.

The answer is:

d. people can get paralytic shellfish poisoning during dinoflagellate blooms

Dinoflagellate algal blooms, during red-tide, release strong neurotoxins that can be ingested by shellfish and passed on to humans who eat the infected shellfish. The most significant public health problem that has been known to  cause this harmful algae poisoning is: Amnesic Shellfish Poisoning (ASP).

as resources become scarce because of population growth, the growth decreases after few years due to lack of resources and food.