Supplementary angles are two angles whose measures add up to 180 degrees. Therefore, if \( m∠b = 121° \), we can find \( m∠a \) using the equation:
\[ m∠a + m∠b = 180° \]
Substituting the value of \( m∠b \):
\[ m∠a + 121° = 180° \]
To solve for \( m∠a \), subtract 121° from both sides:
\[ m∠a = 180° - 121° = 59° \]
So, the measure of angle \( a \) is \( 59° \).
The correct response is:
59°