If you compound twice a year, you need to divide your annual interest rate by 2, and knowing that one thing alone lets you see that A is the right answer here.
But the math in this question is not proper math. The question tells us that Y represents some unknown value. They then write "0.0Y" in the answer choices, where we're meant to assume Y instead represents an unknown digit. It would be perfectly reasonable (correct, in fact) to assume 0.0Y means a product of 0.0 and Y. I say that interpretation is correct because if you read a question that says "k is a single digit integer between 0 and 9, and 2k > 17", then k absolutely does not represent an unknown units digit in the number '2k'. It represents an unknown value- the inequality tells us that 2 times k is greater than 17.
_________________
GMAT Tutor in Toronto
If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com