The correct answer is option 'C', which is 802.5. Let's explore why this standard provides for a collision-free protocol.

A collision occurs in a network when two or more devices attempt to transmit data simultaneously over a shared medium, such as an Ethernet cable. Collisions cause data packets to be lost or corrupted, leading to decreased network performance. To avoid collisions, network protocols employ various mechanisms.

Here's an explanation of the 802.5 standard and how it provides collision-free protocol:

1. IEEE 802.5:
- IEEE 802.5 is a standard for Token Ring LAN (Local Area Network) technology.
- It specifies the physical and data link layer protocols for communication within a token-passing ring network.
- Token Ring networks use a token-passing mechanism to control access to the shared network medium.

2. Token-Passing Mechanism:
- In a Token Ring network, a special frame called a token circulates around the network.
- Only the device possessing the token is allowed to transmit data.
- When a device has finished transmitting, it releases the token, allowing the next device to transmit.
- Thus, collisions are avoided because only one device can transmit at a time.

3. Advantages of Token Ring:
- Token Ring provides a deterministic access method, ensuring that each device has equal opportunity to transmit.
- This results in a collision-free protocol, as only the device with the token can transmit data.
- It guarantees fair access to the network resources, preventing data collisions and ensuring reliable communication.

4. Comparison with Other Standards:
- 802.2: This standard defines the Logical Link Control (LLC) sublayer and does not specifically address collision avoidance.
- 802.3: This standard, also known as Ethernet, uses a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) mechanism to handle collisions.
- 802.6: This standard refers to the Distributed Queue Dual Bus (DQDB) protocol, which provides collision detection and avoidance mechanisms but not a completely collision-free protocol like Token Ring.

In conclusion, the 802.5 standard provides a collision-free protocol by using a token-passing mechanism in Token Ring networks. This ensures that only one device can transmit data at a time, eliminating collisions and ensuring reliable communication.