The most appropriate adverb to fill in the blank is:

(C) never

The reason for choosing "never" as the most appropriate adverb to fill in the blank is as follows:

The original sentence is "He talks ill of his friends," which indicates that the subject, "he," speaks negatively about his friends. To convey the idea that he doesn't engage in this negative behavior at any time, we use the adverb "never."

So, the addition of "never" in the sentence changes its meaning to "He never talks ill of his friends," signifying that the subject refrains from speaking negatively about his friends under any circumstances. This adverb appropriately modifies the verb and conveys the intended meaning.