To determine the correct answer, let's analyze the options:
Option A) 13 - This option is incorrect because Gandhi was not 13 years old when he reached London to become a barrister.
Option B) 15 - This option is incorrect because Gandhi was not 15 years old when he reached London to become a barrister.
Option C) 19 - This option is correct. Gandhi was 19 years old when he reached London to become a barrister.
Option D) 21 - This option is incorrect because Gandhi was not 21 years old when he reached London to become a barrister.
Therefore, the correct answer is option C) 19. Gandhi was 19 years old when he reached London to become a barrister.