Find the digit that is not written in the same font as others. 
Fonts have a recognisable fingerprint: sans-serif digits have plain, uniform-width strokes and no small feet at the ends of the lines, while serif digits add short cross-strokes, called serifs, at the stroke ends, and often vary the stroke width along a curve.
Checking the digits one by one, 1, 2, 3, 4, 6, 7, 8 and 9 all share the same plain, uniform-stroke look, with no extra marks at the ends of their lines. This is the sans-serif fingerprint, and it is consistent across all of them.
The digit 5 breaks this pattern. Its top bar and the base of its curve carry small serifs, and its stroke width is not as uniform as the others. That serif fingerprint does not match any of the other digits.
So the correct answer is 5.













