Answer:
Pay the entire balance due before the grace period expires.
Step-by-step explanation:
The grace period is a period of time people have to pay the full amount of a purchase with the credit card before the finance fee is charged. Banks usually give grace periods between 21 and 25 days. According to this, the best way Elena has to reduce the cost of using a credit card for this purchase is to pay the entire balance due before the grace period expires.
The other options are not correct because they will generate financial charges.