The available option expiration dates for a stock, ascending, with the number of listed contracts at each. Use one of these as the `expiration` for the option chain. This endpoint returns dates and counts, not price quotes; an empty successful response means the stock has no listed options, while an unavailable feed returns 503. Pass `expired=true` to list past expirations instead, for use with the session and bars endpoints; `startDate` and `endDate` then bound the expiration dates and default to the last 90 days.