$\mathrm{Li}, \mathrm{Be}, \mathrm{B}, \mathrm{C}$ - these elements belong to the same period. Generally, the value of 1 st ionisation potential increases in moving from left to right in a period, since the nuclear charge of the elements also increase in the same direction. But the ionisation potential of boron $\left(\mathrm{B} \rightarrow 2 s^2 p^1\right)$ is lower than that of beryllium (Be $\rightarrow 2 s^2$ ), since in case of boron, $2 p^1$ electron have to be removed to get $\mathrm{B}^{+}$, while in case of $\mathrm{Be}, 2 s^2$ electron have to be removed to get $\mathrm{Be}^{+}\left(2 s^1\right)$. $p$ electron can be removed more easily than $s$ electron so the energy required to remove electron will be less in case of boron. The order will be $\mathrm{Li} < \mathrm{B} < \mathrm{Be} < \mathrm{C}$