The correct order of C , N , O and F in terms of second ionisation potential is

Step-by-step solution

Second ionization potential (I₂) is the energy to remove the second electron. After removing the first electron: C⁺ has 2p¹, N⁺ has 2p², O⁺ has 2p³ (half-filled, stable), F⁺ has 2p⁴. C has lowest I₂ as 2p electrons are least tightly bound with lowest nuclear charge. N has slightly higher I₂ than C due to increased nuclear charge. F has highest I₂ due to highest effective nuclear charge. O⁺ with half-filled 2p is relatively stable, making I₂ of O intermediate. Order: C < N < F < O