I'll go with E.
Q:Political candidates' speak with promises that will benefit others, will make people believe that they are the people with good intentions though they have selfish motives behind them. Hence the promises made by them are unreliable.
Assumption : selfish motives = unreliable
A. The argument presumes, without providing justification, that if a person's promise is not selfishly motivated then that promise is reliable.
=> not selfish -> reliable
The argument is talking about selfish motives. Out of scope.
B. The argument presumes, without giving any justification, that promises made for selfish motives are never fulfilled.
=> selfish motives -> never fulfilled
let's keep B, but I'd be careful about 'never' fulfil to replace 'unreliable'.
C. The argument confuses the effect of an action with its cause.
The argument is talking about motives to effect. It does not confuse effect with cause.
D. The argument ignores the fact that a promise need not be unreliable just because the person who made it had an ulterior motive for doing so.
The argument does a promise need to be reliable just because person who made it had an ulterior motive for doing so?
The argument is talking about selfish motive -> unreliable.
Out of scope since the argument does not talk about ulterior motive -> ????
E. The argument ignores the fact that a candidate who makes promises for selfish reasons may nonetheless be worthy of the office for which he or she is running.
This is better choice than B.
By pointing out selfish motive -> may be worthy, it attacks the argument directly and breaks the assumption.
EDIT:
ah, I simplified question too much and made mistake though simplification.
Assumption: promises that benefit others-> selfish motives-> unreliable.
Argument sums up to be candidates who make promises to benefit others are unreliable. what? something is off. that means basically every candidate who promises benefits are unreliable! Then what, we should vote for candidates who will harm the others, or candidates who makes no promises to benefit the community?
E speaks Selfish motives-> may be worthy. This only half-way attacks assumption, although directly.
D: promises needs not be unreliable -> negate-> promises needs unreliable just because person have ulterior motives . Negation technique seems to work. This attacks 'promises that benefit others' part.
candidates can be reliable even if they have selfish motives to run the office.
Is this 700+ Question?