The dot product of two orthogonal vectors is always zero.
Vector u can be expressed (-5,-3) as its ends on this points and starts from origin. The dot product of vector u with vector given in option A results in zero.
(-5, -3) · (1/5, -1/3)
= -5(1/5) + (-3)(-1/3)
= - 1 + 1
= 0
Hence first option is the correct answer
