(c)
Equation of straight line passing through the points
$a_{1}\widehat{\mathbf{i}} + a_{2}\widehat{\mathbf{j}} + a_{3}\widehat{\mathbf{k}}\mathbf{\ }and\ b_{1}\widehat{\mathbf{i}} + b_{2}\widehat{\mathbf{j}}\mathbf{+}b_{3}\widehat{\mathbf{k}}$ is
$a_{1}(1 - t)\widehat{\mathbf{i}} + a_{2}(1 - t)\widehat{\mathbf{j}} + a_{3}(1 - t)\widehat{\mathbf{k}} + \left( b_{1}\widehat{\mathbf{i}} + b_{2}\widehat{\mathbf{j}} + b_{3}\widehat{\mathbf{k}} \right)t$