The defense attorney cross-examined the witness and pointed out

Split #1: subject-pronoun-verb agreement
Discrepancies are plural.

Options D and E incorrectly use a singular verb:

...discrepancies [in her testimony] that WAS . . .

Eliminate D and E

Split #2: ambiguity

Options A and C use identical words. They move the adverb around, but that move does not help. Option B is different.

Why?

A and C suffer from ambiguity. Each could be correct in two ways. Both could be correct in different ways. We have no way to choose.

A) probably implies that the discrepancies contradict earlier testimony "effectively" in the sense of efficiently:

Did the discrepancies capably or competently contradict the earlier statements?

OR

C) probably implies that the discrepancies contradict earlier testimony "effectively" in the sense of essentially[u] or in [u]actuality:

Did the discrepancies basically or essentially contradict the earlier statements?

Further, I could argue that A and C have two ambiguous meanings apiece.
(Put C's probable meaning in A and vice versa.)
I selected "probably" because
unless an author uses a different structure that changes emphasis,
most English sentences have emphasis at the end.
The issue of weighting is much too subtle; it is not tested directly on the GMAT.)

We don't know what to do with A and C.

Eliminate A and C.

Answer B is correct.
"in effect" is an idiomatic phrase.

It has one general meaning: essentially, basically, or (another idiom) "for all practical purposes."

But in effect does not mean capably or efficiently. The phrase is not ambiguous:
The discrepancies, in effect,contradicted the earlier statements.

Similar: By endorsing candidate X who espoused white supremacist beliefs, leader Y, in effect, announced that at the least he had no problem with very public hateful beliefs and speech.

Similar: When the NFL banned peaceful protest and did so despite support for the protest from patriotic armed service members and veterans, the NFL agreed, in effect, with a political leader who criticized the peaceful protest and whose record on civil rights is abysmal.

Eliminate A and C

Experts Global writes incredibly subtle questions -- probably too subtle here.

But you can guess this one by figuring out that there's no way to choose between (A) and (C).

Possible Split #3

Alternatively, although I don't think this argument is as strong, I could argue that...

...discrepancies don't have any human agency. Discrepancies cannot themselves be efficient and capable on the one hand,
or have the effect of rendering earlier testimony useless, on the other.

That argument is strengthened by the passive and impersonal phrase "in effect" in option B.
The phrase "in effect" downplays the doer (the casual agent) and highlights the result or outcome.

By convention, in English, often when we want to stress results and to indicate that the "doer" is an inanimate thing whose result or effect requires human action in the background, we use some kind of passive voice or construction.

*******

Option B decides the issue for us. Its one phrase means one thing.
The sentence makes sense.

Option B is the correct answer.