**Correct option: (D) 90°**

**Explanation:**

P(t) = A [j˄ cos kt – j˄ sin kt]

F = [{dp(t)} / {dt}]

F = (d / dt) A [i˄ cos kt – j˄ sin kt]

= A [– i˄ sin kt (k)– j cos kt (k)]

F = Ak [– i˄ sin kt – j˄ cos kt]

angle between two vectors is θ

cos θ = {(__F__ ∙ __P__) / (|__F__| |__P__|)}

= A [{(j˄ cos kt – j˄ sin kt) ∙ Ak (– i˄ sin kt – j˄ cos kt)} / {|__F__||__P__|}]

= A^{2} k [{– sin kt cos kt + sin kt cos kt} / {|__F__| |__P__|}]

= A^{2}k(0)

= 0

θ = cos^{–1}(0)

θ = 90°