Class A has the marks 50, 56, 62, 68, 74.

Step 1. Class A. The squares are $144, 36, 0, 36, 144$, so the variance is $\dfrac{360}{5} = 72$ and the standard deviation is $\sqrt{72} = 8.49$.

Step 2. Class B. The squares are $9, 1, 0, 1, 9$, so the variance is $\dfrac{20}{5} = 4$ and the standard deviation is $2$.

Step 3. Consistent. Class B: a typical mark is only 2 from the mean.