$\begin{array}{1 1}(a)\;\text{Methanoic acid and 3-methyl-2-butanone}\\(b)\;\text{Methanal and 2-methyl-2-butanone}\\(c)\;\text{Methanal and 3-methyl-2-butanone}\\(d)\;\text{Methanoic acid and 2-methyl-2-butanone}\end{array}$