Which statement describes stereoisomerism?

• A. Different connectivity
• B. Same structure but different spatial arrangement in 3D
• C. Same molecular formula and same connectivity
• D. Different molecular formula

Answer: (B)

Stereoisomerism is when molecules have the same atoms in the same order (same molecular formula and the same connectivity) but differ in how those atoms are arranged in three-dimensional space. Because their 3D arrangement is different, they can have different properties, such as how they rotate plane-polarized light or how they react.

So the statement describing this idea is that the same structure is kept but the spatial arrangement in 3D is different. The other possibilities don’t fit: changing how atoms connect would be a constitutional (structural) isomer; having a different molecular formula isn’t isomerism at all; and having the same formula and same connectivity would be the same molecule rather than a stereoisomer.