I think the correct answer is the last option. Zinc is the softest substance among the choices given. Pure zinc is a soft metal. It is when alloyed it will became hard. Its hardness as a pure metal is only 2.5 using the Mohs scale.
The others materials, they have a bigger value of hardness in Mohs scale, like the CuO with a value of 3.5, the SiC with a value of 9, and Zinc with a value of 2.5.
