It's D.
What we need to prove here is that stocking regular syrup is not a better option than stocking low-calorie syrup.
Imagine that FastMart stores regular syrup: according to D, 75% of its customers will buy the syrup.
On the other hand, provided that FastMart stores low-calorie syrup: 25% of low-calorie lovers will buy it + those 75% who prefer regular syrup but are willing to switch provided that regular is not available. In this way, by storing low-calorie syrup, FastMart attracts 100% of syrup lovers.
I could easily discard A, B and E but I considered C a contender. I made 2 scenarios that helped me eliminate C:
the price of regular can be either equal or lower than that of low-calorie syrup.
Scenario 1: prices of regular and low-cal are equal --> it is better to sell regular because more people will buy it (75%)
Scenario 2: if the price of regular is lower than that of low-cal ---> FastMart will not increase sales by selling regular.
The first scenario strengthens the argument and the second scenario weakens it. Since we have no further information regarding the price, we cannot pick this statement.