Which of the following is not a very hard metal? 

Zinc

The correct answer is option 4. Zinc.

Hardness in metals is a property related to their resistance to deformation, scratching, or abrasion. Here's a brief explanation of the hardness of the metals mentioned:

1. Chromium: Chromium is a hard metal. It is often used in the production of stainless steel, where its hardness contributes to the durability and corrosion resistance of the alloy.

2. Molybdenum:  Molybdenum is also a hard metal. It is frequently alloyed with other metals to enhance their hardness, strength, and corrosion resistance.

3. Tungsten: Tungsten is known for its exceptional hardness. It has the highest melting point of all elements, and it is commonly used in applications where high hardness and temperature resistance are critical, such as in the production of cutting tools.

4. Zinc: Zinc is not considered a very hard metal. It is relatively soft and malleable. While zinc has other valuable properties, such as corrosion resistance, it is not typically chosen for applications where high hardness is a primary requirement.

In summary, while chromium, molybdenum, and tungsten are known for their hardness, zinc is not considered a very hard metal in comparison.