Answer: c. Certificate of deposit.
Step-by-step explanation: As Oliva wants a good rate of interest, so option A and option D will not serve the purpose because savings account offers a small interest rate monthly. Option B will not also be not in the queue because Oliva doesn't need her money from six months to a year and so she can deposit her money for a fixed period of time. Thus, the best choice is to get a certificate of deposit which offers a fixed interest date and has a fixed maturity date.
Thus, the correct option is C. Certificate of deposit.