Intuitively, since 0.64 is something like half-ish (the number is near one half, or 0.5), then the result of 28*0.64 should be near the half of 28.
The half of 28 is 14. But the number 0.64 is greater than 0.5, so the answer should be also greater than 14.
It may be 15 or 21. Calculate the product:
Notice that 17.92 is 3.08 units far from 21, and 2.92 units far from 15.
Therefore, the best estimate for 28*0.64 in the list of options, is 15.