To determine the probable end date of a release in an Agile project, the key inputs considered typically include:

(a) Prioritized User Stories: The order and priority of user stories in the product backlog influence which features will be included in the release.

(b) User Story Estimates in size: Estimations of user story sizes (e.g., story points or other relative size measures) provide insight into the complexity and effort required for each user story.

(d) Team Velocity: The team's historical velocity, which is a measure of how much work the team can complete in a given time frame (usually in story points or similar units), helps in forecasting how many story points can be completed in the release.

So, the correct answer is "Option 4: a, b, c, and d." All of these inputs are valuable for making an informed decision about the probable end date of the release during release planning.