**Core Concept**
The question tests the ability to identify a specific pathogen based on clinical presentation and microbiological findings. The key concept here is the identification of gram-negative diplobacilli, which narrows down the differential diagnosis to a few specific bacterial species.

**Why the Correct Answer is Right**
The presence of large, square-ended gram-negative diplobacilli in the corneal ulcer scraping is characteristic of Moraxella species, particularly Moraxella lacunata. However, in the context of an alcoholic patient presenting with a corneal ulcer, the most likely causative agent is Moraxella lacunata. This bacterium is commonly found in the conjunctiva and cornea of individuals with compromised immunity, such as those who are alcoholic. The square-ended morphology of the bacteria is a distinctive feature that helps in its identification.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pseudomonas aeruginosa is a gram-negative rod that can cause corneal ulcers, but it typically appears as a motile, bipolar-stained rod, not a diplobacillus. Pseudomonas is more commonly associated with contact lens-related keratitis.
* **Option B:** Haemophilus influenzae is a gram-negative coccobacillus that can cause conjunctivitis and keratitis, but it is not typically associated with the presentation of a corneal ulcer in an alcoholic patient.
* **Option C:** Klebsiella pneumoniae is a gram-negative rod that can cause corneal ulcers, but it is not typically described as a diplobacillus.

**Clinical Pearl / High-Yield Fact**
In cases of suspected bacterial keratitis, it is essential to perform a thorough Gram stain and culture of the corneal scraping to guide treatment. The choice of antibiotics should be based on the specific bacterial species identified, and the patient should be closely monitored for signs of progression or treatment failure.

**Correct Answer:** C. Moraxella lacunata.