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The maximum carry over that u can get by adding two digits is 1. So if k can be written as abc (a is 100's place, b is tens place and c is units place digit) then abc + 9 will definetly give a carry over of 1 since c is non zero. so b+1 = 3 and therefore b=2.
From the choice 2, abc + 4 can or cannot give a carry over at units place, so b can be either 1 or 2.