**Core Concept**
Electroretinography (ERG) is a diagnostic tool that measures the electrical activity of the retina in response to light. This technique is crucial for assessing the function of the retina in infants, especially when visual acuity testing is not feasible due to the baby's age or developmental stage.

**Why the Correct Answer is Right**
ERG is the preferred method for testing visual function in infants under 1 year because it provides a quantitative measure of retinal function. This technique involves placing electrodes on the infant's cornea and recording the electrical signals generated by the retina's photoreceptors in response to light stimulation. ERG can detect abnormalities in retinal function, which may indicate conditions such as retinopathy of prematurity or congenital visual pathway defects. The results of ERG can help guide further diagnostic testing and management decisions.

**Why Each Wrong Option is Incorrect**
**Option A:** Snellen's chart is a tool used to measure visual acuity in adults and older children, but it is not suitable for infants due to their limited ability to communicate and follow instructions.
**Option B:** Electrooculography (EOG) measures the electrical potential between the front and back of the eye, but it is not as sensitive as ERG for detecting retinal dysfunction.
**Option C:** Optical Coherence Tomography (OCT) is an imaging technique that provides high-resolution images of the retina, but it is not a functional test of visual acuity and cannot be used to assess the electrical activity of the retina.
**Option D:** Optical Coherence Tomography (OCT) is an imaging technique that provides high-resolution images of the retina, but it is not a functional test of visual acuity and cannot be used to assess the electrical activity of the retina.

**Clinical Pearl / High-Yield Fact**
In infants under 1 year, ERG is a critical tool for detecting retinal dysfunction and guiding further diagnostic testing and management decisions. A normal ERG result in an infant can provide reassurance that the retina is functioning within normal limits, while an abnormal result may indicate the need for further evaluation and treatment.

**✓ Correct Answer: A. Electro retinography.**