Answer:I believe your answer would be option B) dilute. Hope this helps.
Explanation:
Option (a) is correct.
A reducing agent is the one which loses electrons to other substance.
Here, Zn has oxidation number 0 in the L.H.S of the equation, but on R.H.S its oxidation number is +2 i.e. it Zn has donated two of its electrons to

.
Hence, Zn is the reducing agent here.
Your answer is B and the element is Carbon
So what your looking for is matching isotopes. Isotopes are elements that are the same in amount of protons but different in mass meaning different number in neutrons. Because when you add the total protons and neutrons together you get your atomic mass. So this can be written as X=said element, top number above=different atomic mass, bottom number below=atomic number. Hope this help!!
Be careful because answer A has same masses but different atomic numbers so different atoms(elements)!!!