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The NMR (Nuclear Magnetic Resonance) spectrum consists of several protons that absorb at a different frequency. The electronic environment of a particular proton decides the absorption frequency of a particular proton.

A rise in electron density makes the nucleus to be shielded, and the absorption proceeds upfield. A decrease in electron density leads the nucleus to be deshielded, and the absorption proceeds downfield.

Protons situated to the electronegative atom are more deshielded in comparison to the protons situated away from it.

a. The proton connected to the more electronegative or electron-withdrawing atoms is more deshielded or less shielded than the proton located away from it. Hence, these protons absorb downfield.

The protons linked to the carbon atom of alkene, carbonyl groups, and benzene ring also absorb downfield.

In this particular molecule, the proton that is connected to the fluorine atom is more deshielded. Hence the H_b proton absorbs downfield.

b. In compound b, the proton that is alkyl substituted is more deshielded than the other H_a proton. The proton is located closer to the electronegative atom than the proton. Hence, the proton is more deshielded than the H_a proton.