What is the main purpose of using stranded conductors in overhead lines for power transmission? 

The correct answer is option 3):(To increase flexibility)

Concept:

Stranded conductors:

• A stranded conductor consists of several thin wires of a small cross-sectional area called strands.​

• Conductor flexibility is much greater in stranded conductors, making them easier to install. The flexibility of the conductor increase with the increase in the number of strands in the conductor. Hence option 3 is correct.
• Stranded wire tends to be a better conductor than solid wire because the individual wires collectively comprise a greater surface area. It is used when higher resistance to metal fatigue is required. The stranded conductor is formed by twisting the strands together in layers.
• The stranded conductor is formed by twisting the strands together in layers. The strands of each layer are laid in a helical fashion over the preceding layer. This process is called stranding.