Common knoledge tells us that sensible exercise and eating

I agree with U2lover totally - kind of frustrating while this appeared on the NOVA prep test 1

their explanation is tells ... results .. are parallel


I am almost convinced to disregard that explanation

X and Y will result... SVA SC

(C) wins here because "dieting" is used as a noun.

Then again, my SC hit rate has been slipping recently

I have to agree to disregard this question. It is just plain ambiguous. I would have opted for B as it is the only one with the proper subject agreement. On the other hand, the past tense is not necessarily warranted with the context but given the better of the evil, I way prefer that to D where you have to subjectively see the two actions as part of a singular noun X AND Y.
There has certainly been some better examples of collective.
eg bread AND butter...

When I read Nova text before, I was so unhappy about this question and answer.

I think IMHO you are right. C should be winner.

sensible exercise and diet(ing) will result in better health

wanted to bring up 2 things to your attention regarding C

first, "dieting" is different from "diet" -> one may take "dieting" is as restricting yourself from certain foods as it isn't clear whether the gerund implies an action or a thing (diet)... I battle with these gerunds all the time and love it... where is mailtheguru???

second, "WILL"..., courtersy of MGMAT SC, should only be used when the outcome is 100% certain... do we know that it's certain??? what does one consider proper diet, etc... very ambiguous, so "WILL" changes the meaning of the sentence.... this point is supported by the OA choice D, which I believe is wrong for SVA

hope some of you agree