Answer:
Titanium; lowest density
Step-by-step explanation:
The cart would use the same volume of metal, no matter which one it was made of.
If weight were the only consideration, the company would choose the metal with the lowest density.
The values I find for the metals are
Ti 4.5 g/cm³
Fe 7.9 g/cm³
Zn 7.1 g/cm³
Titanium would be the best metal for this application.