Step 1: Understanding sprain treatment.
A sprain occurs when a ligament is stretched or torn, typically as a result of an injury to a joint. The treatment for a sprain usually involves rest, ice, compression, and elevation (R.I.C.E.) to reduce swelling and promote healing.
Step 2: Identifying the correct treatment.
The treatment that should be applied to a sprain is "ice." Applying ice helps to reduce swelling and numb the area to alleviate pain.
Step 3: Conclusion.
Thus, the correct word to fill in the blank is "ice."