The correct answer is option (E)Let us look at all the answer choices to understand why.
(A) Some people think all politicians are rotten.Refer to the first line of the passage. Some (not necessarily all) people think that there are only a few rotten politicians. From this, we can infer that there could also be some people who think that all politicians are rotten. Hence, we cannot definitely say that option (A) cannot be true.
(B) Some politicians are scrupulously honest.Refer to the last line of the passage. The author only claims that someone who is scrupulously honest will not make an
effective politician. He/She could still be a politician, albeit an ineffective one. In other words, there may exist politicians who are scrupulously honest (even if they are not effective politicians!). Hence, we cannot definitely say that option (B) cannot be true.
(C) Some people define a politician’s job as obeying the rules of society.This option choice is out of scope. Because we do not know if any, some or all people define a politician's job as obeying the rules of society. Maybe 0% of all people, maybe 10%, may even be 100%. Obeying the rules of society is simply mentioned as a factor that connects to one being an effective politician. Maybe there exist some people who define a politician''s job this way. We cannot be sure. Hence, we cannot definitely say that option (C) cannot be true.
(D) Some deceitful politicians are ineffective.Refer to the third line of the passage. Now, we only know that to do the job properly (i.e. be an effective politician), one needs to be deceitful. In other words, being deceitful is mandatory to be an effective politician. However, this does not mean that every deceitful politician is also an effective one. There can be a politician who is not effective, despite being deceitful. Hence, we cannot definitely say that option (D) cannot be true.
(E) Some scrupulously honest politicians are effectiveWe have already eliminated the other 4, so this option must be the correct answer

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But let us understand why this is correct. Refer to the last line of the passage. In essence it says that:
If (you are scrupulously honest about obeying the laws of society); then, (you cannot be an effective politician). This means that any and every politician who is scrupulously honest, will have to be an ineffective politician.
Thus, this option that states that some scrupulously honest politicians are effective, will never be true. Hence, we can definitely say that option (E) cannot be true.
Cheers!