Good that you thought of it... unfortunately there is no root word of function... because if you think of it for a long time... function is word...so there's no word go with function, so, if you say functional, the root or base word is function. :):

function

A function instructs the Word program to perform a specific task.

content word is aword,such as noun,verb,or adjective while function is a word ,such as a preposition, a conjuction ,or an article

"Function" derives from Latin.