What source of error in an acid-base titration could lead to a higher reading of the acid concentration?
A Excess water in the conical flask will have no influence whatsoever* (in first order approximation). Since you are determining the the amount of substance of methanoic acid with the titration, the volume is irrelevant. The volume you use to determine the concentration is from when you take the aliquot with the pipette.
This option is therefore incorrect.

B You are correct, the excess water left in the burette will dilute you potassium hydroxide solution, causing you to use more of it, and therefore also reading a higher amount of substance of methanoic acid than would be correct. Obviously this leads to a higher concentration.
This is a correct option.

C Rinsing the pipette with water will cause you analysis solution to be diluted. Therefore the calculated concentration would be lower than the actual concentration.
This option is therefore incorrect.

D Rinsing the conical flask with potassium hydroxide solution will lead to having some of the methanioc acid already neutralised. You will use less of your analysis solution, therefore calculating a lower concentration.
This option is also incorrect.



In order to achieve the highest accuracy, you should 


rinse the burette with the potassium hydroxide solution you use to analyse.
rinse the conical flask with plenty of deionised water (to remove any residues).
rinse the pipette with the solution which is to be analysed.


* If it is too much water, then the volume significantly changes. This will cause the indicator substance to react ever slightly differently, which might lead to you not seeing the equivalence point early enough. This could lead to reading a higher concentration.
However, you eyes are a larger source of error if it is a few drops of water.