Supplementary angles are two angles whose measures add up to 180 degrees. Given that \( m∠b = 121° \), we can find \( m∠a \) using the following equation:
\[ m∠a + m∠b = 180° \]
Substituting in the value of \( m∠b \):
\[ m∠a + 121° = 180° \]
Now, to find \( m∠a \), subtract \( 121° \) from \( 180° \):
\[ m∠a = 180° - 121° = 59° \]
Therefore, the measure of angle \( a \) is \( 59° \).
The correct response is 59°.