The correct answer is option 2: Lime stone.

Cement is a commonly used construction material that is used to bind different materials together. It is made through a process called calcination, where limestone (calcium carbonate) is heated at a high temperature to form quicklime (calcium oxide). This quicklime is then mixed with other materials, such as clay or shale, to form cement.

Option 1: Gypsum is not the main ingredient in cement. Gypsum is sometimes added to cement to regulate its setting time.

Option 3: Clay is another material that is sometimes added to cement to improve its workability, but it is not the main ingredient.

Option 4: Ash, on the other hand, is not a typical ingredient in cement production. It may be used in certain types of cement, such as fly ash cement, but it is not the main ingredient.

Therefore, the main ingredient in cement is limestone or lime stone.