Answer:
Mes freres ont les cheveux bruns.
Step-by-step explanation:
Mes freres is in plural form, so "a" won't work, since it's singular of the verb "Avoir" (To have).
Your brother is not the hair itself ,so , "sont" (to be) won't work either.
Mes freres can be remplaces as "Ils".
If we look into the verb "avoir", the conjugation for "Ils" is "ont" in present.