Here's why:
* Subject: The subject of a sentence is the person, place, or thing that the sentence is about.
* Complete Subject: The complete subject includes the simple subject and any words that modify it.
* Simple Subject: In this sentence, the simple subject is "collection".
* Modifiers: The words "a", "valuable", and "stamp" modify "collection" and are part of the complete subject.
Therefore, the complete subject is "A valuable stamp collection".