Final answer:
Further investigation would be needed for the maternal chlamydial infection.
Step-by-step explanation:
Based on the diagnosis of chlamydial conjunctivitis in the child, further investigation would be needed for the option c) Maternal chlamydial infection. Neonatal conjunctivitis can be transmitted from an infected mother to her child during childbirth, and in this case, the child's mother has been diagnosed with chlamydia. It is important to investigate whether the mother's infection was the source of the child's conjunctivitis and to ensure proper treatment and prevention of further transmission.