Out of the listed actions, only option I (paying bills on time) will help the borrower improve their credit score. Option II (applying for another credit card) may actually have a negative impact on the credit score because it can result in a hard inquiry on the credit report, which can lower the score. Option III (contacting a debt relief agency) may also have a negative impact on the credit score because it can result in a negative mark on the credit report. Option IV (raising the credit limit) may or may not have a positive impact on the credit score, depending on how it is managed.
Therefore, the answer is : paying bills on time.