Full adder using 4:1 mux
I'm not going to give you the answer; merely suggest an approach to figure it out yourself.  

To visualize it better, I suggest restructuring the table an compressing A=0 and A=1 into one row for each condition:

$$
\begin{array}{|c|c|c|c|c|}
\hline B &amp; Ci &amp; A &amp; Cout &amp; Sum \\\hline
  0 &amp; 0 &amp; 0/1 &amp; \text{0/0 or 0} &amp; \text{0/1 or A} \\\hline
  0 &amp; 1 &amp; 0/1 &amp; \text{_/_ or ?} &amp; \text{_/_ or ?} \\\hline
  1 &amp; 0 &amp; 0/1 &amp; \text{_/_ or ?} &amp; \text{_/_ or ?} \\\hline
  1 &amp; 1 &amp; 0/1 &amp; \text{1/1 or 1} &amp; \text{0/1 or A} \\\hline
\end{array}
$$

Fill in _/_ for Cout and Sum when A=0 / A=1. Then replace ? with \$1\$, \$0\$, \$A\$, or \$\overline{A}\$ related to the _/_ results.