Step 1: Build the stack layer by layer.
Start with one close packed layer of atoms and call its positions A, the next layer nestles into one set of the hollows in the first layer, call those positions B, giving a stack that already reads AB.
Step 2: Decide where the third layer goes.
There are only two distinct choices for the third layer, it can sit directly above the original A layer, which repeats the pattern as ABAB and produces the hexagonal close packed structure, or it can sit in the remaining set of hollows that lie above neither A nor B, call those C positions, giving a genuinely new layer.
Step 3: Recognise the repeating unit.
Once the fourth layer returns to the A position, the sequence ABCABCABC keeps repeating, and this three layer repeat is the defining signature of the face centred cubic structure, unlike the two layer repeat of hexagonal close packing, so this stacking sequence points specifically to FCC.
\[ \boxed{\text{Face centered cubic}} \]