You can query Index options using the Financial Market endpoints with the prefix I: along with the Index symbol Here's an example with SPX using our Options Chain endpoint:

https://api.polygon.io/v3/snapshot/options/I:SPX?apiKey=**

For reference endpoints, use the Index symbol without the prefix. Here's an example using our Options Contracts endpoint:

https://api.polygon.io/v3/reference/options/contracts?underlying_ticker=SPX&apiKey=**