**Core Concept**
The question is testing the understanding of the required lens power to visualize the entire retina in ophthalmology. The instrument in question is likely a direct or indirect ophthalmoscope. The retina is a complex neural tissue at the back of the eye, and visualizing it requires a specific lens power to magnify and focus the image.

**Why the Correct Answer is Right**
To view the entire retina, a **+78 diopter** lens is attached to the ophthalmoscope. This is because the retina is a large, curved structure that needs to be imaged in its entirety. The lens power of +78 diopters allows for a sufficient magnification and focal length to visualize the entire retina. This is particularly useful in direct ophthalmoscopy, where the lens is placed close to the eye to obtain a high magnification view of the retina.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide any specific lens power that would be suitable for viewing the entire retina.
**Option B:** This option is incorrect because it is a common lens power used for viewing the anterior segment of the eye, not the retina.
**Option C:** This option is incorrect because it is a common lens power used for viewing the posterior segment of the eye, but it may not provide sufficient magnification or focal length to visualize the entire retina.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the lens power required to view the retina can vary depending on the type of ophthalmoscope used and the individual's eye anatomy. However, a +78 diopter lens is a commonly recommended power for direct ophthalmoscopy.

**Correct Answer:** C. 20 diopters is commonly used for viewing the posterior segment, but it may not provide sufficient magnification or focal length to visualize the entire retina.