I met the two girls, whom I believe, are twin-sisters.

Answer: Option A

Okay, let's break down this sentence improvement question!

The original sentence is: "I met the two girls, whom I believe, are twin-sisters."
We need to see if we can make it better.

The problem lies in the word "whom" and the phrase "are twin-sisters."

Here's why the original sentence is wrong and why the correct answer works:

* Understanding "who" vs. "whom":
Think of "who" as replacing "he/she/they" and "whom" as replacing "him/her/them." In this case, the girls are performing the action of *being* twin sisters. They aren't receiving an action. So, "who" is more appropriate here.

* The Verb "to be":
The phrase "I believe" is called a parenthetical phrase - it add extra information.
It changes the grammatical structure slightly . We need to include "to be" because "believe" often takes an infinitive ("to be", "to do", etc.) when followed by a description.

Now, let's look at the options:

* Option A: who I believe, are
This uses "who" which is a step in the right direction, but it's missing the "to be" part.

* Option B: who I believe to be
This is the correct answer! It correctly uses "who" and includes "to be" after "believe." It makes the sentence grammatically sound.

* Option C: who, to my belief, are
This is grammatically incorrect. While "to my belief" is a valid phrase, it doesn't fit smoothly into the sentence structure and doesn't fix the underlying problem with the verb form.

* Option D: No improvement
We know the original sentence needs improvement, so this is wrong.

In Summary: The best way to rewrite the sentence is: "I met the two girls, who I believe to be, twin-sisters."