which element is likely to be the most brittle?\nselenium\ncobalt\nplatinum\nzinc

Brief Explanations:

Generally, non - metals are more brittle than metals. Selenium is a non - metal (metalloid), while cobalt, platinum, and zinc are metals. Metals have metallic bonding which gives them properties like malleability (ability to be hammered into sheets) and ductility (ability to be drawn into wires), reducing brittleness. Non - metals/metalloids like selenium lack the extensive delocalized electron sea of metallic bonding and are more likely to be brittle.

Answer:

selenium