Plant Growth Regulators

Step 1: Concept
Auxins are vital plant hormones discovered through several historic experiments.
Step 2: Meaning
IAA is the most common natural auxin found in plants.
Step 3: Analysis
While its effects were first studied in oat coleoptiles, the chemical compound Indole-3-acetic acid was first physically isolated from human urine.
Step 4: Conclusion
IAA was first isolated from human urine.
Final Answer:(C)