Which feature in Stripe allows you to set the length of a free trial?

The feature that allows you to set the length of a free trial in Stripe is "trial_period_days." This parameter is specifically designed to enable businesses to offer a defined number of days for which a subscription can be utilized without a charge. By setting this parameter when creating or updating a subscription, you can easily specify how long the trial period will last before billing begins.

This feature is particularly useful for encouraging users to try a service before committing to a payment. During the trial period, customers can experience the full functionality of the subscription without any initial cost. Once the specified "trial_period_days" have elapsed, the subscription will convert to a paid state and the billing cycle will begin based on the subscription plan defined.

Other options listed do not correspond to any known parameters for setting free trial lengths in Stripe, which reinforces why "trial_period_days" is the appropriate choice for this functionality.