Mark against the correct answer:
The two angles of a triangle are complementary. The third angle is

Options

A. `60^(@)`
B. `45^(@)`
C. `36^(@)`
D. `90^(@)`

Answer

To solve the question, we need to find the third angle of a triangle when two angles are complementary. Let's go through the steps:

### Step-by-Step Solution:

1. **Understanding Complementary Angles**:
- Complementary angles are two angles whose sum is 90 degrees.

2. **Setting Up the Triangle**:
- Let's denote the three angles of the triangle as Angle A, Angle B, and Angle C.
- According to the problem, we can say that Angle A and Angle C are complementary. Therefore:
\[ \text{Angle A} + \text{Angle C} = 90^\circ \]

3. **Using the Triangle Angle Sum Property**:
- The sum of all angles in a triangle is always 180 degrees. Hence, we can write:
\[ \text{Angle A} + \text{Angle B} + \text{Angle C} = 180^\circ \]

4. **Substituting the Complementary Angles**:
- Since we know that Angle A + Angle C = 90 degrees, we can substitute this into the triangle angle sum equation:
\[ 90^\circ + \text{Angle B} = 180^\circ \]

5. **Solving for Angle B**:
- To find Angle B, we can rearrange the equation:
\[ \text{Angle B} = 180^\circ - 90^\circ \]
- Simplifying this gives:
\[ \text{Angle B} = 90^\circ \]

6. **Conclusion**:
- Therefore, the third angle (Angle B) is 90 degrees.

### Final Answer:
The third angle is **90 degrees**.