An important finding in Went’s experiment was
A. Unequal distribution of elongation promoting substance in Avena coleoptile
B. Presence of elongation factor in all cells of root
C. Curvature of coleoptiles is proportional to auxin concentration
D. Curvature occurred due to irregular elongation of cells.

Hint:The stem bends toward the light because the side of the stem that is away from the light grows faster than the side of the stem facing the light. This uneven growth pushes the tip toward the light source. The growth of new plant cells is stimulated by the plant chemical auxin.

Complete answer:
To answer this question, first, we need to know about the Went’s experiment. Frits Warmolt Went was a Dutch biologist whose 1928 experiment demonstrated the existence of auxin in plants. His interest gradually shifted to environmental influences on plant growth. At Caltech he was the first to examine the significance of hormones in plant development and growth. He performed a vital role in the growth of synthetic plant hormones that then became the base of most of the chemical agricultural industries.
Now, let us find the solution from the option.
F. W. Went discovered that when the tip of Avena coleoptile is cut down, its elongation stops. However, when auxin was put outside, it started to grow again. He reported that experiments describe that auxin is produced in the coleoptile tip and is moved bottom top the gravity. It moves from the lighter side to the dark side. More auxin is found on the dark side than on the bright side. Because of this, the plant develops more on the darker side and moves in the way of the light source. It depicts positive phototropism where faster cells get elongated toward the darker side and less on the brighter side.

Thus, the correct option is A.i.e. Unequal distribution of elongation promoting substance in Avena coleoptiles.

 Note:Went’s experiment on the growing plants showed that plant grow towards the light in response to a signal at the head portion of the coleoptiles, This signal is a hormone known auxin moves down the dark side of the plant to stimulate faster growth the light.