The correct option is C soldiers
Concrete nouns are nouns which can be experienced through one/more of our five senses viz. sight, smell, hearing, touch and taste.
Option A: The word "We" is a pronoun. Hence, it is not the correct answer.
Option B: The word " salute" is a verb. Hence, it is not the correct answer.
Option C: The word "soldiers" refers to a group of people who fight for their country. Soldiers can be seen with our eyes. Hence, this word is a concrete noun.
Option D : The word" selfless" is an adjective which qualifies/defines the soldiers. Hence it is not the correct answer.