Propose the structure of a compound that exhibits the following ¹H NMR data:

(a) C₅H₁₀O

1.09 δ (6H, doublet)

2.12 δ (3H, singlet)

2.58 δ (1H, septet)

(b) C₅H₁₂O

0.91 δ (3H, triplet)

1.19 δ (6H, singlet)

1.50 δ (2H, quartet)

2.24 δ (1H, singlet)

(c) C₄H₁₀O

0.90 δ (6H, doublet)

1.76 δ (1H, multiplet)

3.38 δ (2H, doublet)

3.92 δ (1H, singlet)

(d) C₄H₈O₂

1.21 δ (6H, doublet)

2.59 δ (1H, septet)

11.38 δ (1H, singlet)