ANS:D - right angled triangle with equal sides.

The most economical section of a triangular channel is typically the isosceles triangle with a 45° vertex angle. In hydraulic engineering, the term "most economical" often refers to the configuration that minimizes construction costs while maintaining hydraulic efficiency. Among the options provided:

• An equilateral triangle has equal sides and equal angles of 60°, making it less economical due to its larger cross-sectional area.
• A right-angled triangle may not be the most efficient since the right angle can result in energy losses and increased friction.
• A right-angled triangle with equal sides would have two 45° angles, but it may not offer the optimal balance of flow capacity and construction costs.

The isosceles triangle with a 45° vertex angle offers a balanced configuration. It provides efficient flow characteristics while minimizing construction materials and costs, making it the most economical section for a triangular channel in many practical applications.