Moving a parent moves all of its subtasks together and Subtasks can be rearranged relative to their sibling subtasks are correct.

When you move or rank a parent issue on the backlog or board the parent and its nested subtasks remain grouped so the whole group moves together and the parent retains its relationship to those subtasks.

You can also reorder subtasks within the same parent so that sibling subtasks appear in a different sequence without having to move the parent itself. This lets you adjust the internal order of work items while keeping them under the same parent.

Subtasks keep fully independent order from their parent is incorrect because subtasks are shown as children of their parent and the parent and its subtasks are treated as a grouped unit when the parent is moved.

You must rank every subtask before you can rank its parent is incorrect because you can rank the parent issue directly and move the entire group without first ranking each individual subtask.

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