The correct order of decreasing first ionisation energy is

On moving from left to right in a period, as the size decreases, ionisation energy increases. But first IE of elements of II A group is higher than that of the elements of III A group because of the stable $n s^2$ configuration (of II A group). Hence, the order of first ionisation energy is ($$\begin{array}{c} & \mathrm{C} & > & \mathrm{Be} & > & \mathrm{B} & > & \mathrm{Li} \\ \text{Group no.} & \text{ (IV) } & & \text{ (II) } & & \text{ (III) } & & \text{ (I) } \end{array}$$)