Concept: Diagonal tacking is a temporary stitch used to hold interfacing and fabric together, especially on curved edges.Explanation:• A round collar is curved, so interfacing must be held firmly in place without shifting. • Diagonal tacking uses small slanting stitches that keep the interfacing smooth and flat. • Even tacking is for temporary joining, and tailor’s tacking is for marking, so they are not the best choice here.Answer: C. Diagonal tacking.