Concept:In a compound noun, the first noun works as a modifier and stays singular, while only the head noun takes the plural form.
Explanation:The word "Girl" is an attributive noun here, not a possessive, so forming "Girls" before "Guide" is grammatically wrong.
That eliminates "Girls Guide" and "Girls Guides" as possible answers.
"Girl Guide" refers to one member of the organization, so it does not fit the idea of joining the body or group.
A person joins the organization, which is commonly called "the Girl Guides."
So the intended sentence becomes: "Susan has been urging me to join the Girl Guides."
This gives the correct combination of a singular modifier and a plural head noun.
Answer:Option A — "Girl Guides."