I found a similar question, so I think this can be taken as our problem because you seem to forgot including the picture. So, for letter a, the answer is: they represent different compounds. As you can notice, the CH₂CH₃ and CH₃ are both on the top side for the left figure, while the CH₃ and CH₃ are on the top side of the right figure. They are cis-trans isomers. The left figure is trans-2-pentene while the left figure is cis-2-pentene.
