How many grams of water react to form 7.21 moles of ca(oh)2

You have the stoichiometric equation. This tells you unequivocally that an  

18

⋅

g

mass of water, 1 mole, reacts with a  

56.07

⋅

g

mass of quicklime to form a  

74.09

⋅

g

mass of slaked lime.

If you don't from where I am getting these numbers, you should know, and someone will be willing to elaborate.

Here, you have formed  

6.21

⋅

m

o

l

of quicklime which requires stoichiometric lime AND water. And thus you need a mass of  

6.21

⋅

m

o

l

×

18.01

⋅

g

⋅

m

o

l

−

1

water  

≅

88

⋅

g

.

In practice, of course I would not weigh out this mass. I would just pour  

100

−

200

⋅

m

L

of water into the lime.

the answer is b because, investigating whether another species of moth also drinks tears might be interesting, but it would not directly explain why this species of moth does.

to explain why this species of moth drinks tears, the scientist could ask: "what nutrients that moths need are in elephant tears? "

the scientist could experiment with moth repellent or oter ways to keep the moths away, but that would not answer his original question.