20 points will mark braniest

excerpt from recommended levels of insulation (excerpt ii)
u. s. environmental protection agency

the type of insulation you should choose depends on how you will use it and on your budget. while closed-cell foam has a greater r-value and provides stronger resistance against moisture and air leakage, the material is also much denser and is more expensive to install. open- cell foam is lighter and less expensive but should not be used below ground level where it could absorb water. consult a professional insulation installer to decide what type of insulation is best for you.

in context, "greater r-value" corresponds with

a)
cheaper insulation

b)
more effective insulation

c)
less effective insulation

d)
more questionable insulation