$a) \dfrac{1}{3} + \dfrac{2}{3} = \dfrac{1+2}{3} = \dfrac{3}{3} = 1$
$b) \dfrac{5}{6} + \dfrac{4}{6} = \dfrac{5+4}{6} = \dfrac{9}{6} = \dfrac{3}{2}$
$c) \dfrac{7}{2} + \dfrac{6}{2} = \dfrac{7+6}{2} = \dfrac{13}{2}$
$d) \dfrac{9}{5} + \dfrac{1}{5} = \dfrac{9+1}{5} = \dfrac{10}{5} = 2$