The car is heading north at a speed of 50 km/h. It takes a left turn 90^o without changing speed. The change in velocity of the car is

For these kinds of queries, vector subtraction and the direction of the input vector and the output vector need to be carefully considered. To better grasp a subject like this, we need make a basic sketch of the question.

Given, V_{i ,} V_f 
Initial velocity = V_i ​=50j
Final velocity =​ V_f =−50i
Change in velocity = Δv=$v_f$ - $v_i$​=−50i−50j
Negative x- means west direction and part y-negative means south direction. The resulting speed is therefore south-west.

Change of velocity =$\sqrt{\left(\right(-{50}^2)+(-{50}^2\left)\right)}$= 70. 71 km/h
Therefore, the answer is 70 km/h and the direction is south-west direction.
The correct option is B.