Structural isomers

Can someone please explain why A is incorrect
What’s the reason?
And please suggest some nice resource from which i can study isomers
from basic


A isn’t branched it’s actually the same linear structure as the original molecule.
I would recommend you try free 3D molecule models online to draw out the structures, you will have an easier time seeing they are the same exact structure, the only difference is the carbons are in a zigzag rather than a perfectly narrow line. But this slight difference doesn’t qualify them as isomers.

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