$a)\frac{1}{4}+\frac{3}{4}=\frac{4}{4}=1$

$b)\frac{3}{5}+\frac{4}{5}=\frac{7}{5}=1\frac{2}{5}$

$c)\frac{5}{7}+1\frac{4}{7}=1\frac{9}{7}=2\frac{2}{7}$

$d)2\frac{5}{6}+3\frac{1}{6}=5\frac{6}{6}=6$

$e)\frac{5}{12}+\frac{7}{12}+\frac{9}{12}=\frac{21}{12}=\frac{7}{4}=1\frac{3}{4}$

$f)2\frac{3}{8}+3\frac{5}{8}+4\frac{7}{8}=9\frac{15}{8}=10\frac{7}{8}$