In a flowchart, we use a ......... to express a decision.a.Diamondb.Ovalc.Rectangled.Polygon

In a flowchart, we use a diamond shape to express a decision. The diamond shape is specifically designed to represent branching points in the process, where a yes/no question or true/false condition is evaluated. When a process reaches a diamond shape, it typically indicates that different paths can be taken based on the answer to the decision posed within the diamond. This visual representation helps to clarify the flow of logic and decisions, allowing viewers to easily understand the potential outcomes based on the choices made. The other shapes listed, such as oval, rectangle, or polygon, have different meanings: ovals represent the start and end points, rectangles represent processes or actions, and polygons are less commonly used. Therefore, the correct answer is:

a. Diamond.