An introduction to organic chemistry: explain question

A student is studying the isomers of two compounds, X and Y.

X has the molecular formula $\ce{C4H9Cl}$. The student draws the structural formulae of two isomers of X, A and B.

A: $\ce{CH3CH2CH2CH2Cl}$ B: $\ce{CH3CHClCH2CH3}$

(a) State the type of isomerism shown by A and B, and explain why they are isomers.

(b) The student also draws two further isomers of X, C and D.

C: $\ce{(CH3)3CCl}$ D: $\ce{CH3CH2CHClCH3}$

State the type of isomerism shown by B and D, and explain why they are isomers.

(c) Compound Y is a cyclic alcohol with the molecular formula $\ce{C4H8O}$. The student suggests that Y can show both geometrical (cis/trans) isomerism and optical isomerism.

Explain why Y can show both types of stereoisomerism, and identify the structural feature of Y that gives rise to each type.