Please Help I Am Confused.

according to the law of conservation of mass"The mass of reactants must be equal to the product"

so by inspection (trail or error method )

the coefficients are

2,1,2 respectively

so

2(2)+1(2)=2(2)+2(1)

4+2=4+2

6=6

d i think its   answer d

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.