Verified answer

1) Aluminium

2) Nickel

3) Sodium

I THINK SO.....................:)

Ni

mineral harness =4.0 (Mohs scale)

Brinell Hardness 700 MN m-2

http://www.webelements.com/nickel/physics.html

Al

mineral harness = 275 (Mohs scale, a log scale)

Brinell Hardness = 245 MN m-2

http://www.webelements.com/aluminium/physics.html

Na

mineral harness = 05 (Mohs scale)

Brinell Hardness = 0.69 MN m-2

So Ni>Al>Na for hardness. Use your experience: a Ni coin is quite hard; an Al rod is easily dented; Na you can cut with a plastic knife.