Asurvey shows that candidate a has a 7% point lead over candidate b. however, the margin-of-error of the poll is +/- (plus or minus) 4%. this means the lead may actually be a)10% (b)4% (c)0% (d)17%

Either A) 10% or B) 4% would work.

Technically, the margin of error of a poll, in this case +/- 4%, would mean the actual populace might be either 4 points higher or 4 points lower than the reported 7% lead.  So it could be as low as a 3% lead or as high as an 11% lead.  So that's an 8 point range within which the actual voters might fall.

Notably, a study reported in the New York Times in 2016 found that most political polls actually have ended up with about a 6 to 7% margin of error compared to actual voting results.  (That's a range of 12 to 14 points total between the minus end and the plus end.)  So you can't always assume that polling results will predict actual election results.

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