Which condition is NOT a cause for removing synthetic web slings from service?

Minor discoloration is the correct answer because it does not typically indicate a loss of functionality or safety in synthetic web slings. Discoloration could occur due to exposure to sunlight, chemicals, or other environmental factors, but as long as the integrity of the sling is intact, it can still perform its intended role.

In contrast, conditions like melting or charring, broken or worn stitches, and knotting used to shorten slings are significant safety concerns. Melting or charring indicates that the sling has been exposed to high heat, which can compromise its tensile strength. Broken or worn stitches can lead to failure during lifting operations, while knotting to shorten slings can create weak points and alter the distribution of load, making the sling unsafe to use. Thus, only minor discoloration does not warrant removal from service.