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# Which of the following rings in most strained ?

$(a)\;Cyclohexane \\ (b)\;Cyclopentane \\ (c)\;Cyclobutane \\ (d)\;Cyclopropane$

Due to the planar geometry of cycloalkanes, the carbon - carbon bond angles between two different bonds in the cycloalkanes deviates from the ideal tetrahedral bond angle $(109^{\circ}28')$.
This deviation introduces a strain in the molecule which is called angle strain.
The greater the deviation from the normal tetrahedral angle, greater is the angle strain and lesser is the stability .
Cyclopropane has the maximum angle strain among the cycloalkanes.
Hence d is the correct answer.