Supplementary angles are two angles whose measures add up to 180 degrees. Given that \( m∠b = 121° \), we can determine \( m∠a \) using the equation:
\[ m∠a + m∠b = 180° \]
Substituting the value of \( m∠b \):
\[ m∠a + 121° = 180° \]
Now, subtract 121° from both sides to solve for \( m∠a \):
\[ m∠a = 180° - 121° = 59° \]
Therefore, \( m∠a = 59° \). The correct answer is:
D. 59°