$\begin{array}{1 1}(a)\;\text{2-ethyl-3-methyl-1-butene}\\(b)\; \text{3-ethyl-2-methyl-3-butene}\\(c)\;\text{2,5-dimethyl-3,4-dimethylhex-3-ene}\\(d)\;\text{3-ethyl-2-methyl-1-butene}\end{array}$
