Explanation:

When a rivet is subjected to tension, the force applied to the rivet tries to pull it apart. The tensile strength of a rivet is the maximum amount of tensile force it can withstand before it fails.

In the case of counter-sunk rivets, the heads are designed to be flush with the surface of the material, which means they are tapered and conical in shape. This allows the rivets to be countersunk or recessed into the material, providing a smooth and flush finish.

When a counter-sunk rivet is subjected to tension, the tapered head distributes the force over a larger area, reducing the stress concentration at the rivet head. This redistribution of force improves the tensile strength of the rivet, allowing it to withstand higher tensile forces compared to a non-counter-sunk rivet.

Effect on Tensile Value:
The tensile value of a rivet refers to its maximum tensile strength. In the case of counter-sunk rivets, the tensile value is increased by 33.3%.

Reasoning:
The counter-sunk head of a rivet reduces the stress concentration at the head, resulting in a more uniform distribution of forces. This reduces the likelihood of failure due to excessive stress concentration at the head of the rivet.

By reducing stress concentration, the counter-sunk head allows the rivet to withstand higher tensile forces before failure. This increase in tensile strength is approximately 33.3% higher than that of non-counter-sunk rivets.

Therefore, the correct answer is option B - the tensile value of a rivet in tension with a counter-sunk head is reduced by 33.3%.