The correct answer is: A. RT-1
RT-1 is a grade of tar that is specifically designed for grouting purposes. It is a bituminous material that is made from coal tar and is a good choice for grouting because it is water-resistant and has good adhesive properties.
RT-2 is a grade of tar that is used for roofing and waterproofing. It is not as water-resistant as RT-1 and does not have as good adhesive properties.
RT-3 is a grade of tar that is used for paving and road construction. It is not as water-resistant as RT-1 or RT-2 and does not have as good adhesive properties.
RT-5 is a grade of tar that is used for industrial applications. It is not as water-resistant as RT-1, RT-2, or RT-3 and does not have as good adhesive properties.
In conclusion, the recommended grade of tar for grouting purpose is RT-1.