The correct answer is option (A):
more sociable than he is
The correct answer is "more sociable than he is".
Let's analyze why. The sentence "You are a great deal more sociable than him" uses a comparative adjective ("more sociable") to compare your level of sociability with someone else's. When making comparisons using "than" in formal English, we need to consider the grammatical role of the word following "than".
In this sentence, "than" is connecting two clauses implicitly. The full comparison is "You are more sociable than he is sociable". Here, "he" is the subject of the implied second clause ("he is sociable"). Therefore, the pronoun following "than" should be in the subject case, which is "he".
Option 1: "more sociable than he is" - This is the most complete and grammatically correct option. It clearly shows the comparison is between "you" (the subject of the main clause) and "he" (the subject of the implied clause).
Option 2: "more sociable than him" - While commonly used in informal speech, "him" is an object pronoun. In formal grammar, when comparing subjects, the subject pronoun ("he") is preferred after "than".
Option 3: "more social than he" - The word "sociable" is an adjective meaning outgoing and friendly. "Social" is also an adjective but refers to relating to society or social organizations. While "social" can sometimes be used to describe a person's behavior, "sociable" is the more appropriate adjective to describe a person's tendency to be outgoing and enjoy company. Additionally, even if "social" were used, the pronoun "he" is still correct as explained above.
Option 4: "No change" - As discussed, the original sentence contains a grammatical error in formal English.
Therefore, "more sociable than he is" is the best choice to replace the underlined part because it correctly uses the subject pronoun "he" after "than" and clearly expresses the intended comparison.