1. Recall the trend of electron affinity
   Electron affinity generally becomes more negative across a period and less negative down a group. However, there are important exceptions due to very small atomic size and electron-electron repulsion.

2. Compare chlorine and fluorine
   Although fluorine is above chlorine in Group 17, chlorine has higher electron affinity than fluorine.

• For fluorine: the added electron enters the very compact $2p$ orbital, where repulsion is high.
• For chlorine: the added electron enters the larger $3p$ orbital, where repulsion is less.

So,

3. Compare fluorine and oxygen
   Both are in the second period, and halogens generally have higher electron affinity than chalcogens because halogens need one electron to attain noble gas configuration.

Thus,

4. Combine the comparisons
   From the above,

5. Check options

• A: $\text{F} > \text{Cl} > \text{O}$ ❌
• B: $\text{F} > \text{O} > \text{Cl}$ ❌
• C: $\text{Cl} > \text{F} > \text{O}$ ✅
• D: $\text{O} > \text{F} > \text{Cl}$ ❌

Therefore, the correct order of electron affinity is