number of squares of diagonals of length 2 units can be formed from the lines y=x+c and y=-x+c where c={0,1,2,3,4}

Only one for c = 1. First of all draw the given lines in Cartesian plane. Diagonals will come out to be x and y axis. Since point of intersection of the diagonals will be origin (0,0).Therefore length of any diagonal will come out to be 2c.Thus, 2c = 2. Hence c = 1.Therefore only one square will satisfy the condition for c = 1.