The correct option is C IAA
The Avena geo-curvature test is a bioassay for auxin-type growth regulators. Etiolated oat coleoptiles are used and the test is conducted in special perspex trays under diffuse daylight. The coleoptiles, with the primary leaves intact, are arranged in grooves on the trays in 4 series of 6 coleoptiles each, and cut to a size of 10 mm. The test substance is applied on an agar strip covering only the lower halves of the apical cut surfaces of each series. After a 4-hour stay in the dark in moist Petri dishes the coleoptile cylinders are shadow graphed on a 35-mm photographic film. The curvatures are measured from an enlarged projection (10 times natural size) of the shadow graphs. The lowest IAA concentration which can be determined is around 30 to 60 μg/l IAA. The concentration response curves follow a logarithmic course up to 1,000 μg/l.