To answer this question, we need to know the age at which Mahatma Gandhi (Gandhiji) got married to Kasturbai.
Option A) 19 years - This option is incorrect because Gandhiji did not get married at the age of 19.
Option B) 12 years - This option is incorrect because Gandhiji did not get married at the age of 12.
Option C) 15 years - This option is incorrect because Gandhiji did not get married at the age of 15.
Option D) 13 years - This option is correct because Gandhiji got married to Kasturbai when he was 13 years old.
The correct answer is D. Gandhiji was 13 years old when he got married to Kasturbai.