This bioassay is used to detect the presence of auxin

The correct option is Split pea stem curvature test and Avena curvature test.

Auxins are a class of plant hormones (or phytohormones) that play an essential role in regulating plant growth, particularly by promoting cell elongation. Bioassays are experimental procedures used to detect and quantify the biological activity of such substances based on their effects on living tissues.

Let us examine the bioassays mentioned in the options to understand why this selection is correct:
1. Avena Curvature Test: This is one of the classic and most widely used bioassays for auxins, originally developed by Frits Went. It uses the coleoptile of Avena sativa (oat). When an agar block containing auxin is placed unilaterally on a decapitated coleoptile tip, the auxin diffuses down one side, causing unequal cell elongation. This results in the bending (curvature) of the coleoptile. The degree of curvature is directly proportional to the concentration of auxin present.
2. Split Pea Stem Curvature Test: This bioassay also measures auxin activity. Sections of etiolated pea stems are split longitudinally down the middle and placed in a test solution. Due to the differential absorption of water and unequal cell elongation in the outer and inner tissues of the split stem in the presence of auxin, the split halves curve inward. The angle of curvature corresponds to the concentration of auxin.
3. Tobacco Pith Culture: While tobacco pith tissue is used in tissue culture experiments to study the cell division and differentiation effects of auxins in combination with cytokinins, it is not primarily classified as a standard diagnostic bioassay dedicated simply to detecting the presence of auxin in the same rapid, quantitative curvature-based manner as the Avena and split pea stem tests.

Therefore, both the Split pea stem curvature test and the Avena curvature test are standard, classic bioassays used to detect and measure the presence of auxin.