What is mixed with oil to create grease?

Grease is created by combining oil with a thickening agent, and soap is the most common of these agents. The process of making grease involves emulsifying oil with soap, which gives grease its characteristic consistency and allows it to adhere to surfaces. The soap's ability to thicken the oil makes it suitable for various applications, providing lubrication that can stay in place under the pressures and conditions found in machinery and other systems.

The other choices do not serve this purpose. Water does not mix with oil to create grease, as the two substances are immiscible. Alcohol can mix with both oil and water, but it does not contribute to the thickening properties necessary for forming grease. Acid is reactive and does not form a stable mixture with oil for lubrication, as it can break down the oil or react negatively with it. Thus, soap is the correct answer, as it plays a crucial role in creating the desired consistency of grease for lubrication purposes.