Counting stereoisomers of C4H8O2 diols with one C=C Steps:

• Unbranched terminal alkene with vicinal OH groups: but-3-ene-1,2-diol has one chiral center, yielding 2 enantiomers.
• Unbranched internal alkene with 1,4-diOH: (E)- and (Z)-but-2-ene-1,4-diol, yielding 2 geometric isomers.
• Branched exocyclic alkene with 1,3-diOH: 2-methylenepropane-1,3-diol is achiral, yielding 1 isomer.
• Unbranched internal alkene with 1,3-diOH: (E)- and (Z)-but-2-ene-1,3-diol, yielding 2 geometric isomers. Why C is correct:
• Molecular formula C4H8O2 (two O from diol, degree of unsaturation matches one C=C) has exactly 7 stereoisomers from 4 structural types per isomerism definitions. Why the others are wrong:
• A: Excludes geometric isomers of but-2-ene-1,3-diol.
• B: Misses one geometric isomer of but-2-ene-1,3-diol.
• D: Includes invalid structures like geminal diols or extra stereoisomers.

Final answer: C