Chlamydia trachomatis: This is the most likely cause of neonatal conjunctivitis, especially given the age of the patient and the symptoms of redness and mucopurulent discharge. Chlamydia is a common cause of conjunctivitis in newborns, typically acquired during passage through an infected birth canal.

Staphylococcus aureus: A common cause of conjunctivitis in all age groups, including neonates. It can cause a range of symptoms from mild to severe.
Streptococcus pneumoniae: Though less common than Chlamydia or Staphylococcus in this context, it is a possible cause of conjunctivitis in neonates and should be considered, especially if there's any suspicion of exposure or if the neonate is not responding to initial treatment.
Haemophilus aegyptius: Can cause conjunctivitis, particularly in children, but it's less commonly associated with neonatal conjunctivitis compared to Chlamydia.

Neonatal Herpes Simplex Virus (HSV): Although not listed among the options, HSV is a critical diagnosis to consider in neonatal conjunctivitis due to its potential for severe morbidity and mortality. It typically presents with vesicular skin lesions, but can also cause conjunctivitis.
Gonococcal Conjunctivitis: Caused by Neisseria gonorrhoeae, this condition is a medical emergency due to the risk of corneal perforation and blindness. It's less common but critical to identify and treat promptly.

Adenovirus: While adenovirus can cause conjunctivitis, it's less common in neonates compared to other age groups and typically presents with additional symptoms such as fever and upper respiratory symptoms, which are not mentioned in this case.