Who from among the following is counted as employed?

a$.$ Frank would like to work but has not looked for work because he believes no jobs are available.

b$.$ Ava has been offered several jobs in the past four weeks but has chosen not to accept any of the offers.

c$.$ Ana Maria spends all of her time taking care of her children at home.

d$.$ Ray wants to work $40$ hours per week but can only find work for $20$ hours per week.