The volume of a parallelopiped formed by $\hat i+a\hat j+\hat k,\:\hat j+a\hat k\:and\:\:a\hat i+\hat k$ is minimum if $a=?$

Question

$(a)\:\:-3\:\:\:\qquad\:\:(b)\:\:3\:\:\:\qquad\:\:(c)\:\:1/\sqrt 3 \:\:\:\qquad\:\:(d)\:\:\sqrt 3$