$a)$

$7,\left(54\right)=7,\underline{5}454\dots$

$7,\left(45\right)=7,\underline{4}545\dots$

$7,\left(54\right)>7,\left(45\right)$

 

 

$b)$

$0,\underline{3}63636\dots <0,\underline{6}36363\dots$

 

$c)$

$0,\left(78\right)=0,787878787878\dots$

$0,787878<0,\left(78\right)$