I compared the two progenies that had .8 +AC/-AC (NY-SF/2 &3) which is equivalent to being sensitive to ammonium chloride as described in the stem. The ratio stayed the same with HA and NA being SF while the rest changed to NY in either 2 or 3. The ratio changed significantly when HA was switched to NY in NY-SF/6(.008) while NA stayed at SF.

This question took me way too long to solve -- too much mental gymnastics, classic NBME.

Well, the easiet way to solve is just look at all the columns for PA , HA, NP ....

In this example, ratio near to 0.8((compare the value of ratio AC+/AC-) means resistance.

Now look at all strains and see which strains has value close to 0.8

After finding that, which gene segment is common among all those strains(so it is HA).