For elements B, C, N, Li, Be, O and F, the correct order of first ionisation enthalpy is:

🧠 Two anomalies in Period 2: Be > B and N > O Across a period, IE normally increases. But two pairs break this rule: Be beats B (filled 2s is more stable than half-filled 2p), and N beats O (half-filled 2p is more stable than paired 2p).

🗺️ Build the order step by step Normal trend: $\ce{Li} < \ce{Be} < \ce{B} < \ce{C} < \ce{N} < \ce{O} < \ce{F}$. Apply anomaly 1: swap Be and B → $\ce{Li} < \ce{B} < \ce{Be} < \ce{C} < ...$ Apply anomaly 2: swap N and O → $... < \ce{C} < \ce{O} < \ce{N} < \ce{F}$. Final order: $\ce{Li} < \ce{B} < \ce{Be} < \ce{C} < \ce{O} < \ce{N} < \ce{F}$. ⚠️ Common mistake Many students only remember one anomaly and miss the other. There are TWO swaps in Period 2: the Be-B swap and the N-O swap. Forget either one and you pick the wrong option.

$\boxed{\text{Answer: (d)}}$