Which of the following examples best describes the signaling theory of education?

a$.$ The hiring manager offers a job to a recent college graduate because education is correlated with natural ability.

b$.$ The hiring manager offers a job to a recent college graduate because the hiring manager has a bias toward people with college degrees.

c$.$ The hiring manager offers a job to a recent college graduate because they are more beautiful than the rest of the applicants.

d$.$ The hiring manager offers a job to a recent college graduate because they are expected to be more productive than other applicants due to their educational attainment.