Which of the following characteristics is important for a sealant backer?

The correct choice emphasizes the importance of impermeability to water in a sealant backer. Sealant backers are used in joints to provide support for the sealant and to control the depth of the sealant bead, ensuring optimal performance. When a backer is impermeable to water, it prevents moisture from seeping behind the sealant, which is crucial for maintaining the integrity and adhesion of the sealant over time. This characteristic helps prevent potential issues such as mold growth, sealant breakdown, and joint failure, as water intrusion can compromise the effectiveness of the sealing solution.

In contrast, while low density, high elasticity, and resistance to UV light may be beneficial properties for various applications, they do not directly pertain to the critical function of preventing water from compromising the sealant's performance. Low density might affect weight and ease of installation, high elasticity could influence the movement the sealant can accommodate, and resistance to UV light matters for exterior applications, but these factors are secondary compared to the necessity for a watertight seal provided by impermeability.