I see this one sat unanswered over the weekend so I'll attempt to explain the best I can.
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1. "liable" is incorrect.
( I looked up this word and found the exact same meaning as "likely")
You're correct, "liable" is not the problem, though a less common in the English vernacular.
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2. "disadvanged" is not idiomatic.
I believe what the point is here is that "at a (severe) disadvantage" and "severely disadvantaged" imply two entirely different things. In this case, being "at a disadvantage" is a comparative observation between either the President and Congress (assuming US politics) or between the President in his/her final two years in office and the President in his/her term before his/her final two years. Being "disadvantaged" implies the President has some long-standing hardship to overcome. Does reaching the final two years of a President's term inhibit his/her ability to legislate? Assuming no other variables are entering, the answer is "no", so an idiom is used to describe the situation.
Example: GMAT test-taker scores 700+ consistently on GMAT exams. However, on the next exam, the test-taker is sick with the flu. The test-taker is "at a disadvantage" while ill but obviously not necessarily "disadvantaged".
Alternatively, the "disadvantaged" test-taker (perhaps learning disability, no formal education, etc.) is in an entirely different boat than the ill test-taker.
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3. "at a disadvantage."
According to "Dictionary.com", it means "in a position that gives one person an advantage over another."
This is an example. Having too little money to spend has put me at a disadvantage with my friends.
Is this correct? Cannot apply this definition to A. If I do, the sentence doesn't make sense to me.
See explanation for #2 - President vs. Congress or President vs self at another point in time
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4. "cannot often" changed the meaning of the original sentence slightly.
I would argue this explanation as incorrect. The second problem I see with C is the omission of "is" following "and". The name for this escapes me (as have most technical terms to describe the English language outside of noun, verb, etc).
Hope this helps.