Answer: Option (a) is the correct answer.
Step-by-step explanation:
Metals are malleable and ductile whereas non-metals are brittle in nature.
Out of the given options cobalt, platinum and zinc are all metals except selenium which is a non-metal.
Being a non-metal, selenium is brittle in nature.
Thus, we can conclude that selenium is likely to be the most brittle out of the given options.