Mark against the correct answer:
A triangle having sides of different lengths is called

Options

A. an isosceles triangle
B. an equilateral triangle
C. a scalene triangle
D. a right triangle

Answer

To solve the question, "A triangle having sides of different lengths is called," we need to analyze the options provided:

1. **Isosceles Triangle**: This type of triangle has at least two sides that are equal in length. Therefore, it does not fit the description of having all sides of different lengths.

2. **Equilateral Triangle**: An equilateral triangle has all three sides equal in length. Hence, this option also does not meet the requirement of having different lengths.

3. **Scalene Triangle**: A scalene triangle is defined as a triangle where all three sides are of different lengths. This matches the description given in the question.

4. **Right Angle Triangle**: A right angle triangle is defined by having one angle equal to 90 degrees. This does not provide information about the lengths of the sides, and thus does not fit the description.

Based on the analysis of the options, the correct answer is:

**Scalene Triangle** (Option 3).

### Step-by-Step Solution:
1. **Identify the types of triangles**: Understand the definitions of isosceles, equilateral, scalene, and right angle triangles.
2. **Evaluate each option**:
- Option 1 (Isosceles): Has at least two equal sides.
- Option 2 (Equilateral): All sides are equal.
- Option 3 (Scalene): All sides are different.
- Option 4 (Right Angle): Has one angle of 90 degrees but does not specify side lengths.
3. **Determine which option fits the description**: The only triangle that has all sides of different lengths is the scalene triangle.
4. **Conclude the answer**: The correct answer is Option 3 - Scalene Triangle.