What indicates that two option contracts are in the same series?

For two option contracts to be in the same series, they must share the same expiration date and the same strike price. When both of these criteria are met, the contracts are considered to be in the same series, which allows traders to compare them directly in terms of pricing and trading strategies.

In the context of the options market, having the same expiration month and strike price means that they represent the same underlying asset and are thus interchangeable referencing a specific investment position at a specific time. This allows traders to make more informed decisions since they can analyze the same underlying asset under the same conditions.

The other choices indicate variations that would classify option contracts as being from different series—having different expiration dates or different strike prices would clearly define them as separate series. Additionally, being traded on different exchanges does not inherently define whether contracts are in the same series, as the same contract can be listed on multiple exchanges.