Grignard answers

(a) propan-1-ol (3 carbon atoms)
This is a primary alcohol so use methanal. (1 carbon atom)
We need two more carbon atoms in the grignard reagent so start with bromoethane.

(b) propan-2-ol  (3 carbon atoms)
This is a secondary alcohol so use an aldehyde.  Ethanal has two carbon atoms.
We need one more carbon atom in the grignard reagent so start with bromomethane.

(c) butan-2-ol (4 carbon atoms)
This is a secondary alcohol so use an aldehyde.  Ethanal has two carbon atoms.
We need two more carbon atom in the grignard reagent so start with bromoethane.

(d) 2-methylpropan-1-ol (4 carbon atoms)
This is a primary alcohol so use methanal. (1 carbon atom)
We need three more carbon atoms in the grignard reagent so start with 2-bromopropane.

(e) 2-methylpropan-2-ol. (4 carbon atoms)
This is a tertiary alcohol so use a ketone.  Propanone has three carbon atoms.
We need one more carbon atom in the grignard reagent so start with bromomethane.