UPSC NDA Math Model Paper 4

The X-axis makes angles 0°, 90° and 90° with X, Y and Z-axis respectively.
So direction cosine of X-axis:
cos 0 = 1, cos 90 = 0, and cos 90 = 0
So, the sum of the squares of direction cosines of x-axis =12+0+0=1
Hence, option (3) is correct.