Alright, so the correct answer is (B). Let me walk you through why.
Think about the argument's core idea: "We have a corruption problem, so let's create a new, independent group to fix it."
The whole plan depends on one huge, unstated belief: that this new group will actually be the solution. Now, what's the biggest, most obvious way this plan could fail? It would fail if the new group, the "solution," also became corrupt. If your watchdog is just as bad as the people it's supposed to be watching, you haven't solved anything.
That's why (B) The independent agency will itself not fall prey to corruption is the perfect answer. It's the essential assumption that must be true for the plan to work. If you add this sentence into the original argument, it plugs the biggest potential hole in the logic. It basically says, "Don't worry, the solution will actually remain a solution," which makes the conclusion that it is the solution much more believable.
Why the others aren't as good:
(A) The appointment of the independent body is the only way to combat corruption. This is too extreme. The argument says this plan is the solution, but it doesn't have to be the only possible solution in the world. Maybe raising salaries or increasing transparency could also work. This option is too strong and isn't necessary for the argument to be reasonable.
(C) Corruption is not present outside the political class and the bureaucracy. This is completely off-topic. The argument is focused on fixing corruption among politicians and bureaucrats. Whether your local shopkeeper is also corrupt has no bearing on whether this specific plan for the government will work.
(D) The punishment meted out by the investigating agency will not act as a deterrent... This one actually does the opposite! It weakens the conclusion. If the punishment doesn't stop others from being corrupt, then the new agency isn't a very good "solution" to the problem of increasing incidents, is it?
(E) If not controlled immediately, the problem of corruption can spiral out of control. This just adds a sense of urgency. It tells us why we need a solution fast, but it doesn't give any support to the idea that this specific plan is the right one. It's like saying "We need to put out this fire!" but it doesn't help prove that a particular fire extinguisher is the right tool for the job.
So, when you look at it, only (B) directly addresses and strengthens the core logic of the proposed plan itself, making it the best answer.
Bunuel
The country has recently been shaken by the increase in incidents of corruption amongst the political class and the bureaucracy. The solution clearly is to appoint an independent investigating body headed by a person of repute who can investigate such cases of corruption and punish those found guilty.
The conclusion above would be more reasonably drawn if which of the following were inserted into the argument as an additional premise?
(A) The appointment of the independent body is the only way to combat corruption.
(B) The independent agency will itself not fall prey to corruption.
(C) Corruption is not present outside the political class and the bureaucracy.
(D) The punishment meted out by the investigating agency will not act as a deterrent for people/agencies susceptible to corruption.
(E) If not controlled immediately, the problem of corruption can spiral out of control.