What does a 2kg block of solid iron have twice as much compared to a 1kg block of solid iron?

The answer is mass. Compared to a 1kg block of solid iron, a 2kg block of solid iron has twice as much mass. Inertia would not be the correct answer because inertia is a measurement of motion. The question does not mention movement so inertia is incorrect.

Volume would not be correct either because volume measures how much is inside of something or how much a container can hold. A common example of volume is how much water a glass can hold. This measurement would be in liters (L) or milliliters (mL). This leaves the correct answer, which is mass.