Suppose you forget properties of transpose at exam time, you can quickly solve such questions using simple examples:
Let A= $\begin{bmatrix} 1\\2 \end{bmatrix}$ B= $\begin{bmatrix} 3\\4 \end{bmatrix}$ and C= $\begin{bmatrix} 5\\6 \end{bmatrix}$
A'+B'+C'= $\begin{bmatrix} 1 & 2 \end{bmatrix}$ + $\begin{bmatrix} 3 & 4 \end{bmatrix}$ +$\begin{bmatrix} 5 & 6 \end{bmatrix}$ =$\begin{bmatrix} 9 & 12 \end{bmatrix}$
A+B+C =$\begin{bmatrix} 9\\12 \end{bmatrix}$ and (A+B+C)'= $\begin{bmatrix} 9 & 12 \end{bmatrix}$
C is the answer