Explanation:

- Outstanding subscription refers to amounts that are due but not yet received from subscribers, common in memberships or publications.

- Option 1: Income - Money received regularly, especially for work or through investments. Outstanding subscriptions are expected revenue but not received yet, so not current income.

- Option 2: Asset - A resource with economic value that an individual, corporation, or country owns. Outstanding subscription represents money expected to be received, qualifying it as an asset.

- Option 3: Liability - A company's legal financial debts or obligations during business operations. Outstanding subscriptions are not obligations but expectations, so it's not a liability.

- Option 4: None of these - This option would apply if none of the given categories fit, which is not the case here.