The mean is calculated as:
\[
\text{Mean} = \frac{\text{Sum of all elements}}{\text{Number of elements}}
\]
Given numbers: \( 5, 3, 0.5, 4.5, a, 8.5, 9.5 \), mean = 7. Hence:
\[
7 = \frac{5 + 3 + 0.5 + 4.5 + a + 8.5 + 9.5}{7}
\]
Simplifying:
\[
7 = \frac{31 + a}{7}
\]
Multiply by 7:
\[
49 = 31 + a
\]
Solve for \( a \):
\[
a = 49 - 31 = 18
\]
Therefore, the correct answer is 12.