The definition of an angle uses the undefined term .

a. line
b. point
c. plane
d. ray

Point.

Further explanation

This is one of the classic problems of Euclidean geometry.

The angle is determined by three points, we call it A, B, C, with A ≠ C and B ≠ C.

We denote an angle with three points and a symbol ∠.  The middle point is always the vertex.  We can also give angle names only with vertices. For example, based on the attached image, the angle can be denoted as ∠BAC, or ∠CAB, or ∠A.

Types of Angles  

The acute angle represents an angle whose measure is greater than 0° and less than 90°.The right angle is an angle that measures 90° precisely.  The obtuse angle represents an angle whose measures greater than 90° and less than 180°.  The straight angle is a line that goes infinitely in both directions and measures 180°. Carefully differentiate from rays that only runs in one direction.

Note:

Undefined terms are the basic figure that is not defined in terms of other figures. The undefined terms (or primitive terms) in geometry are a point, line, and plane.

These key terms cannot be mathematically defined using other known words.

A point represents a location and has no dimension (size). It is labeled with a capital letter and a dot.A line is an infinite number of points extending in opposite directions that have only one dimension. It has one dimension. It is a straight path and no thickness.A plane is a flat surface that contains many points and lines. A plane extends infinitely in all four directions. It is two-dimensional. Three noncollinear points determine a plane, as there is exactly one plane that can go through these points.  Learn more  Undefined terms are used to define a ray linkDefinition of the line segment link  What are three collinear points on a line? link  

Keywords: the definition of an angle, the undefined term, line, point, line, plane, ray, endpoint, acute, obtuse, right, straight, Euclidean geometry