A unit vector along the vector $4\hat{i} - 3\hat{k}$ is : (a) $\frac{1}{7}$4\hat{i} - 3\hat{k}$$ (b) $\frac{1}{5}$4\hat{i} - 3\hat{k}$$ (c) $\frac{1}{\sqrt{7}}$4\hat{i} - 3\hat{k}$$ (d) $\frac{1}{\sqrt{5}}$4\hat{i} - 3\hat{k}$$