Is this question from gmatplus or OG?
I was wondering A should be the correct answer.
Hereis my thinking,
If we simplify the sentence, it will become as follows:
While some propose to combat 'X' by 'A', others by 'B' and still others by 'C'.
Here A, B and C represent actions.
This looks fine to me. Here the purpose of all the three groups of people is same: "to combat X". So the parallelism should be abserved between the actions by which all the groups of people want to achieve that purpose.
In E, we are basically trying to observe parallelism between
some propose..., others suggest..., and still others are calling
First of all as both of you have mentioned, the last part does not satisfy the requirement of parallelism.
But even if we ingnore that part, if we consider it a case of parallelism, the sentece would tend to suggest the following:
While some people propose to combat X (achieving goal) by action A whereas others suggest B (an action) and still others are calling C (another action)
This version does not show that all the groups of peopl are trying to achieve the same purpose. It suggest that some people propose to combat X while others suggest something else.
If this is the question from GMATPLUS, could it be possible that the given answer could be wrong?