To answer this question, let's go through each option to understand why it is correct or incorrect:
Option A) Monu - This option is incorrect because it is not Gandhiji's nickname in childhood.
Option B) Sonu - This option is incorrect because it is not Gandhiji's nickname in childhood.
Option C) Mahu - This option is incorrect because it is not Gandhiji's nickname in childhood.
Option D) Manu - This option is correct because it is Gandhiji's nickname in childhood.
The correct answer is D. Gandhiji's nickname in childhood was Manu.