For the first option:
If you can buy 3 lb of pears for $3.87 then you can buy 1 lb of pears for 3.87÷3=$1.29
Forbthe second option:
If you can buy 5 lb of pears for $6.65 then you can buy 1 lb of pears for 6.65÷5=$1.33
As we can see, it is cheaper to buy from the first seller as 1 lb of pears is 4 cents cheaper to buy than the second option. So, 3 lb of pears for $3.87 is a better buy.