Right option is (c) \((S_f cos\theta) \vec{e}\)
The best explanation: Here, a right-angled triangle is formed by the vectors \(\vec{E_f}, \vec{S_f}\, and\, \vec{T_f}\). The \(\vec{T_f}\) vector is orthogonal to the \(\vec{E_f}\) vector in this case. The relation is given by \((\vec{e}.\vec{S_f})\vec{e}=(S_f cos\theta)\vec{e}\).