(a) decreases by 2
When every term is decreased by 2, the new mean = old mean − 2, so the mean also decreases by 2.
If original mean is $\bar{x} = \dfrac{\sum x_i}{n}$, and each term becomes $(x_i - 2)$, new mean $= \dfrac{\sum(x_i-2)}{n} = \bar{x} - 2$. The mean shifts by exactly the same constant subtracted from every term. Examiners expect you to state the correct option and give a one-line reason.