The infinitive to is not dispensed with in the second instance of the sentence.
It is just omitted, to be understood by the reader. This is known as ellipsis.
So the two forms of the sentence are identical, technically and semantically.
Both of them are correct.
[Edit] Pl. read in conjunction with my comments below re: and / but.
Here is the sentence that the poster asks about:
The main conclusion of this study was that there is no one, or five, or even ten such failures which, once removed, would boost the development of the sub-sector.
As nearly as I can tell, the intended meaning of that sentence is essentially this:
The main conclusion of this study was that removing one, five, or even ten such failures would not boost development of the sub-sector.
Grammatically there is nothing wrong with using "no one" instead of "not one" in th original wording, although that choice does invite readers to take the wrong path and imagine initially that "no one" is being used as a synonym for "nobody." Likewise there is nothing grammatically wrong with including the "or" before "five," although it tends to prolong the time that readers who took the wrong turn at "no one" will remain on the wrong path before figuring out their mistake.
I do, however, see a logical problem with the wording, "there [are] no ... such failures which, once removed, would boost the development of the sub-sector." That wording amounts to saying that no removed failure would boost development of the sub-sector. But in my opinion, looking at the effect of "a failure, once removed," isn't the same as looking at the effect of "the removal of a failure." After all, you wouldn't say, "once removed, those failures boosted development of the sub-sector."
Ultimately, the crucial question to ask about the original sentence isn't whether the author's convoluted wording contains any grammatical errors. From a practical perspective, the issue is whether readers can make sense of what the author is saying without dedicating undue effort to the task—and judged by that standard, the original wording (especially in print) is very poor. My reworded version (in the second block of boxed text above) conveys the intended meaning in fewer words, without inviting misinterpretation or requiring readers to iron out the author's iffy logic.
Best Answer
It is questionable whether letter a) should be correct. I would argue that the answer is, in fact, c)
The possessive apostrophe in parents' which answer a) contains, is redundant. We have the preposition "of" which tells us that the "owners" of these two friends are the speaker's parents, i.e. the parents of the speaker.