A mark imprinted on paper that is visible when it is held to the light, usually a sign of quality is referred to as:

A watermark is a recognizable image or pattern embedded into the paper that becomes visible when held against the light. This feature is often associated with high-quality paper and is used as a security measure to help authenticate documents and deter counterfeiting. Watermarks are commonly found on currency, legal documents, and certificates to indicate their integrity and authenticity. The presence of a watermark signifies that the paper has been produced with care, often suggesting a higher value or significance compared to standard paper.

The other options do not encapsulate this specific definition or purpose. Bonding refers to the process of adhering materials together, while a security script generally pertains to specific written instructions or codes, and a heat-sensitive bond relates to materials that change properties when exposed to heat. None of these terms accurately describe a visible mark on paper that serves as an indication of quality like a watermark does.