TBH, immediately after reading the q-stem I identify this as a POE type question. What I mean is that the q-stem is vague as to what's actually going on, and it's a scenario that we can't study for. The only important thing to take from the question stem is that we pick an answer where frequency increases since higher pitch ~ higher frequency.
For what it's worth, the medium might be changing slightly to helium, but the person who hears the sound won't be in a helium filled room so the medium will still be normal air in general.
(a). Assuming the medium is the same, increased wavelength would mean decreased frequency. This would be wrong. Even if we assumed the medium changed, meaning frequency is the same, this would still be wrong.
(c). This says frequency decreases which would be wrong.
(d). This statement is incorrect and speed does not have to do with pitch.
Pick (b) because it's "least wrong". Unsatisfying, I know. But that's basically the MCAT in a nutshell, especially CARS.
