$ \text{A.} $ $\frac{1}{2}(\boldsymbol{b}+\boldsymbol{c}-\boldsymbol{a})$
$ \text{B.} $ $\frac{1}{2}(\boldsymbol{a}+\boldsymbol{b}+\boldsymbol{c})$
$ \text{C.} $ $\frac{1}{2}(\boldsymbol{a}-\boldsymbol{b}+\boldsymbol{c})$
$ \text{D.} $ $\frac{1}{2}(\boldsymbol{a}+\boldsymbol{b}-\boldsymbol{c})$