Step 1: What the question asks.
We need the metal that switches on the enzyme IAA oxidase in plants.
Step 2: What IAA oxidase does.
IAA is the natural auxin, indole-3-acetic acid. When the plant has too much auxin, the enzyme IAA oxidase breaks it down. So this enzyme controls the auxin level.
Step 3: Why an activator is needed.
Many enzymes do not work alone. They need a metal ion to become active. That metal is called an activator.
Step 4: The right metal.
For IAA oxidase, the activator is manganese, written as $Mn$. So manganese turns this enzyme on and helps regulate auxin.
Step 5: Why not the others.
Zinc ($Zn$) helps make tryptophan, the auxin precursor, but it is not the IAA oxidase activator. Iron ($Fe$) and molybdenum ($Mo$) are linked to chlorophyll synthesis and nitrogen metabolism, not to this enzyme.
Step 6: Conclusion.
Therefore the correct answer is "Mn". \[ \boxed{\text{Mn}} \]