In what scenario would you use a modified thin-set mortar?

Using modified thin-set mortar is particularly advisable when installing large format tiles or in wet areas due to several important characteristics. Modified thin-set mortar includes additional polymers that provide enhanced performance, which is crucial for ensuring strong adhesion and flexibility.

When dealing with large format tiles, the weight and size increase the potential for movement and stress on the substrate, making the extra flexibility of modified thin-set beneficial. In wet areas, such as bathrooms or kitchens, managing moisture is essential. Modified thin-set helps prevent issues like mold and mildew growth and provides a stronger bond that withstands the challenges posed by water exposure.

The other scenarios mentioned may not require the same level of performance that modified thin-set provides. For instance, small mosaic tiles often use unmodified mortars that can adequately support their lightweight nature. Dry, indoor environments typically do not impose the same challenges as wet environments or large tiles. Re-grouting existing tile installations doesn't necessitate the enhanced properties of modified thin-set mortar, as the existing tiles may not engage with the same requirements for strength and flexibility as new installations. Thus, option B accurately reflects the appropriate use of modified thin-set mortar in specific installation conditions.