IUPAC name: 3-Phenylprop-2-ene-1-al.

Here, the function group is aldehyde. The number of the longest carbon chain attached to the functional group is three. There is a double bond between the carbon atom at position 2 and position 3 with respect to the functional group and a phenyl group is attached to the carbon atom at 3^rd position. Thus, IUPAC name of the above compound is 3-Phenylprop-2-en-1-al.

(ii) IUPAC name: Cyclohexanecarbaldehyde

Here, the function group is aldehyde. The number of the carbon atom in the longest chain attached to the functional group is six and is present in a cyclic manner. Thus, IUPAC name of the above compound is Cyclohexanecarbaldehyde.

(iii) IUPAC name: 3-Oxopentan-1-al

Here, the functional groups are ketone and aldehyde. The number of carbon atoms in the longest chain is five. We know, the naming is done in an alphabetical order. Thus, IUPAC name of the above compound is 3-Oxopentan-1-al.

(iv) IUPAC name: But-2-enal

Here, the function group is aldehyde. The number of the longest carbon chain attached to the functional group is four. There is a double bond between the carbon atom at position 2 and position 3 with respect to the functional group. Thus, IUPAC name of the above compound is But-2-enal.