A

To find a unit vector in the direction of the vector \( \vec{v} = (3, 4) \), we first calculate its magnitude.
Step 1: Magnitude of the vector \( \vec{v} = \sqrt{3^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5. \)
Step 2: The unit vector \( \hat{v} \) is given by \( \hat{v} = \frac{\vec{v}}{||\vec{v}||} = \left( \frac{3}{5}, \frac{4}{5} \right). \)
Therefore, the unit vector in the direction of \( \vec{v} \) is \( \left( \frac{3}{5}, \frac{4}{5} \right) \).