Concept:The correct option is the one that expresses "no matter how much effort was used" before the clause "they tried."
Explanation:The sentence says the workers could not move the heavy stone. It then adds that this was true even though they tried with effort.
The phrase “however hard” means “to whatever degree of effort” or “no matter how much effort.”
In this structure, “hard” describes the effort put into trying, so “however hard they tried” fits correctly.
Option A, “how much,” would need a noun after it, such as “how much effort.”
Option B, “as much,” is incomplete because it needs a comparison like “as much as they tried.”
Option C, “no matter that,” does not suit the idea of effort.
Therefore, the complete sentence is: “The workers could not move the heavy stone however hard they tried.”
Answer:D. however hard