Explanation:

To form a triangle, we need three points. With four points on a circle, we can select any three points and form a triangle. Let's consider each possible selection of three points.

Selection 1:
We select three consecutive points on the circle. Let's label these points as A, B, and C.

- Triangle ABC: This is a valid triangle.

Selection 2:
We select three non-consecutive points on the circle. Let's label these points as A, B, and C.

- Triangle ABC: This is a valid triangle.

Selection 3:
We select two consecutive points on the circle and one non-consecutive point. Let's label these points as A, B, and C.

- Triangle ABC: This is a valid triangle.

Selection 4:
We select one consecutive point on the circle and two non-consecutive points. Let's label these points as A, B, and C.

- Triangle ABC: This is a valid triangle.

Selection 5:
We select three points that are equidistant from each other on the circle. Let's label these points as A, B, and C.

- Triangle ABC: This is not a valid triangle as all three points are collinear.

Selection 6:
We select four points that are equidistant from each other on the circle. Let's label these points as A, B, C, and D.

- Triangle ABC: This is not a valid triangle as all three points are collinear.
- Triangle ABD: This is not a valid triangle as all three points are collinear.
- Triangle ACD: This is not a valid triangle as all three points are collinear.
- Triangle BCD: This is not a valid triangle as all three points are collinear.

So, out of the six possible selections, only four selections result in valid triangles. Therefore, the correct answer is option 'C' - 4 triangles can be formed by joining four points on a circle.