D).To one who knew young Gussie as well as I did, the words opened up a wide field.
Explanation:
Aunt Agatha was a 'strong-minded and dominating woman' and she wanted that Bertie must go to his cousin Gussie in order to rescue him. The line 'To one who knew young Gussie as well as I did, the words opened up a wide field' reflects that Bertie would do as per Aunt Agatha's wish.' The description like 'wide-field' reflect that she employed her powerful personality and influenced Bertie enough to visit Gussie in America. Hence, <u>option D</u> is the correct answer.
