Auxin was isolated from tips of  : 

Coleoptiles of oat seedlings by F.W. Went .

The correct answer is Option (4)- Coleoptiles of oat seedlings by F.W. Went.

The discovery of each of the five major groups of PGRs have been accidental.

All this started with the observation of Charles Darwin and his son Francis Darwin when they observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism).

After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile .

Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings. 

(Experiment used to demonstrate that tip of the coleoptile is the source of auxin. Arrows indicate direction of light)