Let's see here...

a. Unless you're a plant, you shouldn't be very worried about volcanic eruptions.

b. Glacier melting = rise in sea = animals/people move away from the coast.  Most likely, nothing dies, except a few weeds in the sand/rocks near the sea.

c. Albeit being somewhat scary, I wouldn't sweat this.  The suns rays are deadly, yes, but in this context, this isn't what you're looking for.

d. This, I think, is the correct answer.  A sudden reduction in the said animal's food supply would lead to a decline in the population.  Then when the said animal's population declines, the food supply of said animal would increase, since there would be less animals eating it, leading to a sort of up-down pattern.  (See https://isleroyalewolf.org/overview/overview/at_a_glance.html)

But, then again, I'm just spitballing here.  This is purely just intuition, my answer is probably wrong, but hey, you asked us, and I'm just answering

Charles