I'm surprised Hugo hasn't already chimed in here, but everything about this question and answer choices is, at best, misleading.
What's stored in the plan cache is THE execution plan. It's the actual execution plan used to execute the query. It's also the execution plan returned for the Explain/Estimated execution plan. They are one and the same. So the answer of "Actual" is true, and the answer of "Actual and Estimated" is also true, since they're the same.
Now when you request an "actual execution plan" in SSMS, it also displays some of the run-time statistics from that specific execution. Those statistics are not part of the plan cache because they change with every execution (thus contradicting the term "cache"). So perhaps the author chose "Estimated" as the answer because run-time statistics are not stored in the plan cache, but if that's what the question was supposed to ask, it is worded very poorly. From a technical point of view, "Estimated" isn't the wrong answer, it's just one of the three possible correct answers.