Cell is one which performs a specific
function in the body.

"They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body's hereditary material and can make copies of themselves. Cells have many parts, each with a different function"

Explanation:

there is the link

https://ghr.nlm.nih.gov/primer/basics/cell

i think it would be b. tissue because an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.