My father-in-law is blind . . . . . . . . one eye.

The correct answer is (A) of.
Let's break down why:

* When someone is completely unable to see, we usually say they are blind.
* However, when someone has lost sight in one eye, we use the preposition "of" to describe the specific area of blindness.
* Think of it like saying someone is lacking sight *of* one eye.

Let's look at why the other options don't work:

* (B) on: We don't typically use "on" with blindness.
* (C) in: While "in" can relate to locations, it doesn't fit the correct grammatical structure here. We'd need to rephrase the sentence significantly.
* (D) from: "From" often indicates a cause or origin. It doesn't describe the *location* or *extent* of the blindness in this context.

Therefore, "My father-in-law is blind *of* one eye" is the most grammatically correct and commonly used phrase.