Valitania's long-standing practice of paying high salaries to its elec

We need to weaken the conclusion of the argument.

Conclusion: Valitania's practice of paying high salaries has brought wrong kind of people (people interested in money and not serving the nation) in politics.

(A) Many Valitanian candidates for elected office spend some of their own money to finance their campaigns.
Doesn't weaken the conclusion. They could spend their own money to get elected so that they could make much more.

(B) Most Valitanian elective offices have four-year terms.
Irrelevant.

(C) No more people compete for elected office when officeholders are paid well then when they are paid poorly.
Our focus is on the type of people who compete for elected office, not number of people. We could assume that the people who want to serve the nation will compete no matter how they are paid and when paid well, the greedy ones would be additional and hence, if the same number of people compete then it means that only the loyals are competing - but all this is a lot of assumption. Let's see if we have a more direct answer.

(D) Only politicians who rely on their offices for income tend to support policies that advance their own selfish interests.
Does not weaken our conclusion. We don't know how many politicians rely on their office income and hence advance their own selfish interests. If a large majority of officeholders rely on their office income, they would be furthering their own interest and hence there would be wrong kind of people in politics.

(E) Most of those who are currently Valitanian politicians could have obtained better-paid work outside politics.
This tells you that most politicians could have obtained an even higher salary outside. This means that they are in politics not for their salaries but for something else. This weakens our conclusion that high salaries attract people who get lured by the high salary.

Answer (E)