Step 1: Recall what succinic acid looks like.
Succinic acid is a 4-carbon dicarboxylic acid: \( HOOC\text{-}CH_2\text{-}CH_2\text{-}COOH \). It has two \( -COOH \) groups at each end.
Step 2: Understand hot acidified KMnO4 action on cycloalkenes.
Hot acidified \( KMnO_4 \) cleaves the C=C double bond. For a cyclic alkene, ring cleavage places a \( -COOH \) at each carbon of the double bond, giving a linear dicarboxylic acid.
Step 3: Work back from succinic acid.
Succinic acid has 4 carbons total. The two \( -COOH \) groups came from the double bond carbons. So there are 2 internal carbons between them, meaning the ring has 4 carbons total. A 4-carbon ring alkene is cyclobutene.
Step 4: Verify cyclobutene gives succinic acid.
\[ \text{cyclobutene} \xrightarrow{\text{hot, acidified } KMnO_4} HOOC\text{-}CH_2\text{-}CH_2\text{-}COOH \] Oxidative cleavage of the double bond in cyclobutene gives succinic acid.
Step 5: Rule out other options.
Cyclopentene (5C) gives glutaric acid (5C). Cyclopropene (3C) gives malonic acid (3C). 2-Butene is acyclic and gives two molecules of acetic acid. None gives succinic acid.
Step 6: State the answer.
X is cyclobutene. \[ \boxed{\text{Cyclobutene}} \]