A is correct because after inserting child Cases the after insert trigger is fired again for them, infinitely

D. Child case is created for each parent case in Trigger.new.

This means that for every new Case inserted (each entry in Trigger.new), a corresponding child Case is created and inserted with the same Subject and the ParentId set to the parent’s Id.

trigger CaseTrigger on Case (after insert) {
List<Case> childCases = new List<Case>();
for (Case parent : Trigger.new) {
Case child = new Case (ParentId = parent.Id, Subject = parent.Subject);
childCases.add(child);
}
insert childCases;
}

A is OK

A is correct