**Core Concept**
Crede's method is a prophylactic treatment for ophthalmia neonatorum, a conjunctivitis that occurs in newborns. This method involves the application of a specific substance to prevent the disease. The World Health Organization (WHO) recommends this method for its effectiveness in preventing ophthalmia neonatorum.

**Why the Correct Answer is Right**
Crede's method involves applying a 2% silver nitrate solution to the conjunctiva of newborns immediately after birth. The mechanism behind this method is the antimicrobial properties of silver nitrate, which effectively kills the causative pathogens of ophthalmia neonatorum. The silver ion (Ag+) has a broad spectrum of activity, including against Neisseria gonorrhoeae and Chlamydia trachomatis, two common causes of ophthalmia neonatorum. The application of silver nitrate solution creates a chemical barrier that prevents the entry of these pathogens into the conjunctiva, thereby preventing the development of ophthalmia neonatorum.

**Why Each Wrong Option is Incorrect**
**Option B:** Erythromycin ointment is an antibiotic that is used in the treatment of ophthalmia neonatorum, not as a prophylactic agent. It is not the correct answer because it is not used in Crede's method.
**Option C:** Tetracycline ointment is another antibiotic that may be used in the treatment of ophthalmia neonatorum, but it is not the correct answer because it is not used in Crede's method.
**Option D:** Penicillin injection is not used in the prophylaxis of ophthalmia neonatorum. While penicillin is an effective antibiotic, it is not the correct answer because it is not used in Crede's method.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the use of silver nitrate solution in Crede's method has been largely replaced by other antimicrobial agents, such as erythromycin ointment, due to its potential toxicity and the development of antibiotic resistance. However, it remains an important historical landmark in the prevention of ophthalmia neonatorum.

**✓ Correct Answer: A. Silver nitrate solution**