An isomer is a compound that has the same molecular formula of another one, but has different arrangement. So to find if these compounds we have to see if they have the same molecular formula.
Let's see compound number one. It has 5 atoms of C and 12 atoms of H. Its formula is C₅H₁₂.
The second compound has 4 atoms of C and 10 atoms of H. Its molecular formula is C₄H₁₀.
The third compound has 5 atoms of C and 12 atoms of H. Its formula is also C₅H₁₂.
So compound 1 and 3 are isomers since they have the same molecular formula.
Answer: I and III only.