**Explanation:**

A flange coupling is a type of mechanical coupling used to connect two shafts together. It consists of two flanges that are bolted together, with each flange attached to one shaft. The key is a small rectangular piece of metal that is inserted into a keyway on the flange and the shaft. Its purpose is to transmit torque between the flange and the shaft, ensuring that they rotate together.

The key is subject to various types of loads and stresses during operation. These loads can cause the key to fail, leading to a loss of coupling connection and potential equipment failure. Among the different types of loads, shear, tension, torsion, and bending are the most common.

**Shear:**
- Shear load refers to the force that acts parallel to the cross-sectional area of the key.
- In a flange coupling, the key experiences shear load due to the torque transmitted between the flange and the shaft.
- The shear stress on the key can reach critical levels and cause failure if the key is not properly designed or if the torque exceeds its capacity.

**Tension:**
- Tension load refers to the force that acts to stretch or elongate the key.
- In a flange coupling, tension load may occur if there is misalignment between the flange and the shaft, causing the key to experience tension stresses.
- However, tension load alone is not likely to cause key failure in a flange coupling because the key is usually designed to withstand tension stresses.

**Torsion:**
- Torsion load refers to the twisting force applied to the key.
- In a flange coupling, torsion load is the primary load that the key is designed to withstand.
- The key is specifically designed to transmit torque between the flange and the shaft, and it is usually made of a material with high torsional strength.

**Bending:**
- Bending load refers to the force that causes the key to bend or flex.
- Bending load may occur in a flange coupling if the key is not properly supported or if there is misalignment between the flange and the shaft.
- However, the bending load on the key is typically smaller compared to shear and torsion loads.

Based on the above explanations, it can be concluded that a key connecting a flange coupling to a shaft is likely to fail due to shear load (option A). Shear load is the primary load that the key experiences in a flange coupling, and if the shear stress exceeds the key's capacity, it can lead to key failure and loss of coupling connection.