After graduating from high school, i had to think (seriously) about what kind of career i wanted to pursue.
what is the part of speech of the word in parenthesis?

How can behavioral science be applied in improving public health? a.by tracking a disease to determine its sourceb.by creating promotions that improve health habitscboth by tracking a disease to determine its source and by creating promotions that improve health habitsd.neither by tracking a disease to determine its source or by creating promotions that improve health habits