Five-year-old Liam has been acting strange lately. He clings to his mother and expresses jealous feelings toward his father, almost as if his father is a rival for his mother's love. Freud would suggest that Liam is experiencing:

the Oedipus complex                            

Explanation:

The Oedipus complex is also referred to as 'Oedipal complex' is described as a term that was proposed by Sigmund Freud in the 'psychosexual stages of development theory'. It was an attempt made to understand a child's feelings of urges and desires towards his or her parent possessing opposite sex as that of him i.e, a boy's feelings of desires towards his mother and however, consisting anger and jealousy towards the same-sex parent i.e, a boy towards his father.

As per the question, Freud would suggest that Liam is experiencing the Oedipus complex.