What must be done to the mortar before using it during masonry construction?

In masonry construction, it's important to properly prepare mortar to ensure it performs effectively when laid. The correct option emphasizes that before applying mortar, it should be slightly moistened or "sprinkled lightly with water." This action helps keep the mortar workable and can improve adhesion to the masonry units.

Mortar can dry out quickly, especially in warm or windy conditions. Lightly sprinkling it with water helps prevent it from becoming too stiff, allowing for better placement and manipulation. This technique not only enhances workability but also aids in achieving a good bond between the masonry units.

The other options involve methods that either do not enhance the mortar's effectiveness or are not standard practices for preparing mortar. For instance, mixing it with water thoroughly is essential for creating mortar, but applying it immediately after mixing is usually the norm without letting it sit exposed to air. Letting it sit can lead to premature hardening, making it unusable. Heating mortar to increase viscosity is not a typical practice as it could lead to adverse effects on the mortar's properties, such as affecting its curing process or structural integrity.