A part of a line that starts at one endpoint and extends forever in one direction is called a ray.
Here's a detailed breakdown of what each term represents:
A. Ray: A ray has one endpoint from which it extends infinitely in one direction. It is denoted using two points, with the first point being the endpoint. For instance, if the ray starts at point A and passes through point B, it is written as \( \overrightarrow{AB} \).
B. Angle: An angle is formed by two rays (called the sides of the angle) that share a common endpoint (called the vertex). It is not a part of a line; instead, it is a figure formed by two rays.
C. Line: A line is straight (no curves), has no thickness, and extends infinitely in both directions. It does not have an endpoint.
D. Point: A point is a precise location or position in a plane. It has no dimensions (length, width, or height).
So, the correct answer is:
A. ray
