A

Thirty seconds of etching with 37 % orthophosphoric acid removes roughly 10–20 µm of enamel. Experimental work has shown resin tag/etch depths of about 15 µm after the routine 30-s orthodontic etch (Bhandari et al., 2019, PMID 31198341). Legler et al. measured ~27 µm after 60 s with the same acid; halving the time gives an estimated depth near 13 µm, again falling in the 10–20 µm range (Legler et al., 1990, PMID 2198801). Therefore, option A best represents the expected etch depth.