Brochure: Convert your landscape to a water-conserving landscape - you reduce your outdoor water use, its natural and attractive, and it also saves you money.

Criticism (against the brochure claim of 'saves you money'): Conversion would save less than twenty dollars on a homeowner's yearly water bills. This saving cannot justify the expense of new landscaping.
(Say new landscaping costs $1000 which is an extra expense one needs to change. The saving per year on water bill would be just $20 per year so even in 30 years, it will add up to only $600. Hence, the added expense right now is not justified. With the regular yard, current expense required is nothing)

We need to rebut the criticism. So we need to say why changing to water-conserving landscape does save you money.

(A) Even homeowners whose yards do not have water-conserving landscapes can conserve water by installing water-saving devices in their homes.

We need to rebut the criticism which focusses on saving money.

(B) A conventional landscape generally requires a much greater expenditure on fertilizer and herbicide than does a water-conserving landscape.

Here is another reason why water-conserving landscape will help save money. So the $600 saving could become $1200 (just as an example). Hence, this helps rebut the criticism.

(C) A significant proportion of the residents of the city live in buildings that do not have yards.

Irrelevant

(D) It costs no more to put in water-conserving landscaping than it does to put in conventional landscaping.

The point is that CHANGING the yard will cost money. The argument talks about conventional landscaping getting REPLACED by water saving one. If both landscaping cost the same, it is a good idea to install water saving one in the beginning. But changing will require $1000 extra. Hence it doesn't help rebut the criticism.

(E) Some homeowners use more water to maintain their yards than they use for all other purposes combined.

Irrelevant.

Answer: B