1. The problem is to evaluate the expression $5^2 - 2^2$. 2. Recall the formula for squaring a number: $a^2 = a \times a$. 3. Calculate each square separately: $$5^2 = 5 \times 5 = 25$$ $$2^2 = 2 \times 2 = 4$$ 4. Substitute these values back into the expression: $$5^2 - 2^2 = 25 - 4$$ 5. Perform the subtraction: $$25 - 4 = 21$$ 6. Therefore, the value of $5^2 - 2^2$ is $21$. 7. The correct answer is option D: 21.