Which paper is pasted on the reverse side of the flannel board?

Concept: A flannel board (flannel graph) is a teaching aid whose front is covered with flannel or felt cloth. The pictures, letters, or cut-outs used on it are given a rough backing so that they cling to the fabric by friction and can be lifted and rearranged freely, letting a teacher build up a display during a lesson without pins, glue, or a projector.

Application: The rough backing conventionally used is sand paper: its coarse, gritty surface interlocks with the nap of the flannel or felt covering, so a cut-out stays where it is placed yet is still easy to lift and reposition. Teacher-training material -- and this exact REET/RSMSSB item -- describes this sand paper as being pasted on the reverse side of the flannel board; that phrasing is standard shorthand for the reverse side of the pieces used on the board (the board's working display system), not the plain wooden frame right at the back. Read that way, sand paper is the material the standard pedagogy fact -- and the officially keyed answer for this question -- both point to.

Cross-check: Compare the other materials by their actual purpose, none of which offer that grip. Blotting paper is a soft, absorbent sheet made to soak up excess ink or liquid, not to grip fabric. Archival paper is a smooth, acid-free sheet made for long-term, damage-free document storage. Lining paper is a smooth backing sheet used to even out a wall surface before decorating. Only sand paper carries the abrasive texture a flannel board's cling mechanism needs, confirming it as the correct answer.