The correct option is
D Akbar
In order to fill in the blank with a proper noun, let's understand what a proper noun is. Specific names of people, places or things are called proper nouns. For example, Raj loves chocolates. Here, 'Raj' is the name of a specific man and hence 'Raj' is a proper noun.
Option A - 'Sita' is the name of a female. Though it is a proper noun, we can't use 'Sita' here, as the sentence is about a king and not a queen. Hence, option A is incorrect.
Option C - 'Rajputas' is a collective noun, as it represents a sub-caste. Hence, option C is incorrect.
Option D - 'Soldier' is a common noun. Here, the name of a specific soldier is not given, hence it is not a proper noun. Thus option D is incorrect.
Option B - 'Akbar' is the name of a person and he was a great king as well. 'Akbar' is a proper noun. Hence, option B is the correct answer.
'Akbar was a great king who will always be remembered by the people of our country.'