A Linear Regression of Age (X) on Blood Lead Levels

A linear regression of age (x) on blood lead levels (y) for men who have worked in factories that manufacture car batteries is performed. The residual plots suggest there is still a pattern remaining, and you decide to add a quadratic term (age²) for age into the model. Which of the following models is now most appropriate?

A) Blood lead levels = a + b21 (age) + e, e ~iid N(0, s2)
B) Blood lead levels = a + b1 (age) + e2, e ~ iid N(0, s2)
C) Blood lead levels = a + b1 (age) + b2 (age2) + e, e ~iid N(0, s2)
D) Blood lead levels = a + b1 (age2) + e, e ~iid N(0, s2)