Step 1: Understand the question.
We need the scientist who explained the enzyme-substrate complex using the Induced-fit idea.
Step 2: Two famous models.
There are two pictures of how an enzyme grabs its substrate. The old one is Lock and Key, the newer one is Induced-fit.
Step 3: Lock and Key.
Emil Fischer gave the Lock and Key model, where the active site is a fixed shape and only the matching substrate fits, like a key in a lock.
Step 4: Induced-fit.
Koshland improved this. He said the active site is flexible. When the substrate comes near, the enzyme slightly changes its shape and wraps around the substrate to fit it well.
Step 5: Eliminate others.
Michaelis is known for enzyme kinetics, the Michaelis-Menten equation, not this model. Northrop is famous for crystallizing enzymes. So neither gave the Induced-fit idea.
Step 6: Conclusion.
Therefore the correct answer is "E. Koshland". \[ \boxed{\text{E. Koshland}} \]