Step 1: Calculating degree of unsaturation.
For C₄H₆, DU = (2×4 + 2 - 6)/2 = 2.
Step 2: Interpreting the given constraint.
One unsaturation arises from a ring and one from an endocyclic double bond.
Step 3: Enumerating cyclic possibilities.
Constitutional isomers meeting these criteria include cyclobutene and positional variants of methylcyclopropene, totaling three distinct structures.
Step 4: Excluding stereoisomers.
Only constitutional (structural) differences are counted, not geometric or optical isomers.
Step 5: Conclusion.
Thus, the total number of cyclic isomers is 3.