(c) 15
Using the empirical relationship: $3 \text{ Median} = \text{Mode} + 2\text{ Mean}$
$= 21 + 2(12) = 21 + 24 = 45$
$\therefore \text{Median} = 45 \div 3 = \mathbf{15}$
Source: Chapter 13, Section 13.4 (Remarks)
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The key formula to remember is the empirical relationship: $3 \text{ Median} = \text{Mode} + 2\text{ Mean}$. This is stated in the Remarks of Section 13.4. Substituting directly gives Median = 15. Examiners expect students to recall and apply this formula correctly — no derivation needed for an MCQ.