In his upcoming lecture in Prague, the tenured professor plans to

Official Explanation:

it's not clear how the sentence should be corrected. The sentence is too long and complex to try and correct the original sentence, so we will have to use the answers. (D) and (E) use the present tense speaks when future tense is required. The phrase as well as in answer (C) is unnecessary, which leaves us with (A) and (B). In (A) for is the wrong preposition to follow to speak, as the professor is not speaking on behalf of someone or for the advocacy of something, but is speaking about something; therefore, the correct preposition is on, which means (A) is incorrect. (B) uses the correct parallel structure.

A. In his upcoming lecture in Prague, the tenured professor plans to speak for the damaging effects of melting ice caps and depleting water sources and oxygen levels,
plans to speak for - you dont plan to speak for, rather you will plan to speak on something ->out

B. In his upcoming lecture in Prague, the tenured professor plans to speak on the damaging effects of melting ice caps, depleting water sources and oxygen levels,
Correct Answer

C. In his upcoming lecture in Prague, the tenured professor plans to speak on the damaging effects of melting ice caps as well as depleting water sources and oxygen levels,
as well as -> is not used to show parallelism. -> Out

D. Damaging effects of melting ice caps are what the tenured professor speaks on in his upcoming lecture in Prague and also depleting water sources and oxygen levels,
and also redundant -> Out

E. The damaging effects of melting ice caps are what the tenured professor speaks on in his upcoming lecture in Prague, as well as depleting water sources and oxygen levels,
as well as -> is not used to show parallelism -> Out

I don't know whether this is an acceptable GMAT question. If depleting is a modifier in this context, then I think there should be no comma before "depleting" else we distort the meaning of the sentence (implying that the professor is the one depleting water sources and oxygen levels).

One of the thing make me chose C is because there is no comma (,) before AND in the B option making it weird as a sentence, can anybody help?

Speak for - Representing (Team, Organization etc)
Speak on - To give a lecture

In his upcoming lecture in Prague, the tenured professor plans to speak on

the damaging effects of melting ice caps,
depleting water sources and oxygen levels,

all of which are undeniable signs of global warming.

Correct Answer must be (B)

I think you're right. Going through the question once more, I realized the absence of the comma makes the intended list of items of discussion difficult to identify.

Hi Everyone, I will request to clarify a query.

I didn't do with B because I couldn't figure the parallel teams.

B. In his upcoming lecture in Prague, the tenured professor plans to speak on the damaging effects of melting ice caps, depleting water sources and oxygen levels,

Here I understand the three parts: damaging effects of melting ice caps, depleting water sources and oxygen levels, are meaning wise aren't parallel to each other. Instead of oxygen levels, if we would have used depleting oxygen levels, then I guess it would have been parallel.

Can someone help me understand that where I went wrong?