mgmt_gmat wrote:The commission's office of compliance, inspections, and investigations plans to intensify
its scrutiny of stock analysts to investigate not only whether research is an independent
function at brokerage firms, but also whether conflicts result when analysts own the
stocks they write about or when they are paid for their work by a firm's investment
banking division.
A. to investigate not only whether research is an independent function at brokerage
firms, but also whether conflicts result when analysts own the stocks they write
about or when they are
B. to investigate not only whether research is an independent function at brokerage
firms, but also if conflicts result when analysts own the stocks they write about or
they are
C. to not only investigate whether or not research is an independent function at
brokerage firms, but also if conflicts result when analysts own the stocks they
write about or are
D. not only to investigate whether or not research is an independent function at
brokerage firms, but also whether conflicts result when analysts own the stocks
they write about or are
E. not only to investigate whether research is an independent function at brokerage
firms, but also whether conflicts result when analysts own the stocks they write
about or when

ceilidh.erickson wrote:This question is from GMATPrep:

The commission's office of compliance, inspections, and investigations plans to intensify its scrutiny of stock analysts to investigate not only whether research is an independent function at brokerage firms, but also whether conflicts result when analysts own the stocks they write about or when they are paid for their work by a firm's investment banking division.

A. to investigate not only whether research is an independent function at brokerage firms, but also whether conflicts result when analysts own the stocks they write
about or when they are
B. to investigate not only whether research is an independent function at brokerage firms, but also if conflicts result when analysts own the stocks they write about or
they are
C. to not only investigate whether or not research is an independent function at brokerage firms, but also if conflicts result when analysts own the stocks they
write about or are
D. not only to investigate whether or not research is an independent function at brokerage firms, but also whether conflicts result when analysts own the stocks
they write about or are
E. not only to investigate whether research is an independent function at brokerage firms, but also whether conflicts result when analysts own the stocks they write
about or when

To your question:

First, can someone introduce how to use whether? I hear some people say that if whether-clause is in the place of object, we need to cut off or not; if whether-clause is in the place of adverbial modifier, or not cannot be removed. Is this correct?

Yes, you're absolutely right! When the whether-clause is acting as an object, we can generally replace it with "if," so "whether or not" is redundant.

If we use "whether... or not..." as an adverbial modifier, it's no longer redundant. Consider:
I will go to the park, whether it rains or not.

Second, in choice E, can I use if to replace whether?

You could, but only if you replaced both "whethers" with "ifs" for parallelism. Generally, though, the GMAT will prefer "whether" to "if" in this context.