Step 1: Understand the meaning of "yardstick".
"Yardstick" is a figurative term used to denote a standard of measurement or benchmark used for comparison or judgment.
Step 2: Analyze the options.
STANDARD: A level of quality or attainment used as a reference — directly matches the definition.
STATISTICS: Numerical data — not necessarily a reference measure.
MARK: Can be vague; not always a measure or standard.
SIZE: Indicates physical dimension — not a benchmark or reference.
Therefore, the correct answer is STANDARD.