(a) N or O
O has more negative electron gain enthalpy than N. This is because nitrogen has stable configuration with half filled 2p orbitals. So, it will resist gain of electrons whereas oxygen has outer electronic configuration of 2p4. So, it will try to attain stability by gaining 2 electrons.
(b) F or Cl
Cl has more negative electron gain enthalpy than fluorine. This is because the incoming electron repel each other in fluorine due to small size but in chlorine, the incoming electrons due to not face the electronic repulsion due to large size of chlorine than fluorine.