What a disastrously wrong QOTD!
The official answer (about types) is a small part of the correct answer, and the genuinely correct answer is another of the answer options! One doesn't even need to change the stored type to fix the stupid typing, just changing the literal division from 100 to 100.00 has exacty the same effect as tthat with no table type changes, but still delivers wrong results, because the way the calculation is expressed subtracts the absolute value of the percentage (eg for 5 per percent 0.05) insted of the product
Forcing decimal instead of integer arithmetic won't deliver the correct answer, because the given calculation is just plain wrong. Changing the literal divisor from 100 to 100.00 (which forces everything to be converted from int to decimal so that the whole calculation works in decimal) fices the type issue without needing to fix the silly types in the table. But fixing the type issue doesn't deliver correct results. The calculation shown subracts the absolute value of the percentage, not the value of the product of the percentage with the total percentage value.
Or to express it bluntly in tidy notation: X-D/100.00 is usually not the same number as X * (1-D/100.00) and it is just plain crazy for a QOTD to claim that those two calculations deliver the same result.
So to date, 30 % of answers have been correct and been awarded no points, while 70 percent of answers have been wrong (because changing the type doesn't fix the problem) and have been awarded points. It looks as if the majority need to be taught elementary arithmetic, but are being encouraged not to learn it, while the minority who understand it are taught that it is wrong.