Answer:
smooth as silk
Step-by-step explanation:
We are looking for a phrase that shows why Zama thinks the invention is clever and moves with the slightest push. Notice these phrases appear in the last sentence of the passage:
"What a clever invention, I marveled − it has a handle and rolls along smooth as silk at the slightest push!"
Here, "smooth as silk" means the drum rolls very smoothly, without any trouble. So that phrase is the correct answer.