During experiments with shoot tips' response to light, the Darwins concluded that phototropism is caused by , Boysen-Jensen added that phototropism is caused by , and Went further added that phototropism is caused by .

D. a signal; a chemical; auxin                      

Explanation:

Phototropism is described as a phenomenon that involves the growth of a specific organism as an outcome towards a light stimulus. It is generally observed or found in plants yet occurs in a few other organisms, for example, fungi. The different cells that are being found on the plants that are placed far from the light posses a specific chemical known as 'auxin' that generally reacts when the "phototropism" occurs.

tbh idk same thing here

answer; collective security;