Sentence:  Since in a finite group the <WSD>order</WSD> of an element must be a divisor of the order of the group, therefore "o" ("a") cannot be 3 and so we must have "o" ("a")=4=the order of the group "G". 
Meaning of the word marked with <WSD> tags:  For given group "G" and element "g" ∈ "G", the smallest positive natural number "n", if it exists, such that (using multiplicative notation), "gn = e", where "e" is the identity element of "G"; "if no such number exists, the element is said to be of '''infinite order''' (or sometimes '''zero order''')."