Ans=B

Given

$\overrightarrow{a}=3\hat{i}-4\hat{j}$

unit vector in the direction of vector $\overrightarrow{a}$ is given by $\overrightarrow{a}=\frac{\overrightarrow{a}}{\left| \overrightarrow{a} \right|}$

$\hat{a}=\frac{3\hat{i}-4\hat{j}}{\sqrt{3^{2}+\left( -4 \right)}^{2}}$

$\hat{a}=\frac{3\hat{i}-4\hat{j}}{5}$

A vector in the direction of vector $\overrightarrow{a}$ that has magnitude 10 is givin by 10$\hat{a}$

10$\hat{a}$=$6\hat{i}-8\hat{j}$