Am genuinely interested in all the answers above and just trying to apply my ATPL knowledge to the question. Please keep your corrections to my flawed interuption coming.
It is my understanding that Buys Ballot law is a general principle and is independant of height - it applies as a general principle to both upper and lower level winds. Buys Ballot is credited with the law as he was the first to derived it empirically, the law having already been deduced theoretically by the U.S. meteorologist William Ferrel.
The way the question is asked in itself gives certain things away. The tell you what direction the wing is coming from which means that they expect you to know Buys Ballot and indeed if you apply that law the low is indeed ahead of you. They give you a flight level and not a height of FL180. Flight level by definition is a pressure altitude and is the height above the 1013.2 level assuming ISA conditions. So why is the answer not that OAT is increasing as you are actually decending through the athmosphere?.