Step 1: Set up the picture of a two part sand mould.
A standard sand mould is built inside a box called a flask, which is split into an upper section and a lower section that are clamped together, with the pattern impression shared between the two.
Step 2: Assign the standard names to each section.
The lower half of the flask, which sits on the moulding floor, is called the drag, while the upper half, which sits on top and is lifted off to remove the pattern or place cores, is called the cope. If a mould needs a third, intermediate section, that extra piece is called a cheek, and it goes between the cope and drag rather than being the topmost part itself.
Step 3: Answer the specific question asked.
Since the question asks specifically for the topmost part of the mould, and the cope is by definition the section that sits above the drag (and above any cheek, if present), the cope is the correct name for the top most part in sand moulding.
\[ \boxed{\text{cope}} \]