=======================================================
Explanation:
The big X represents some placeholder for another letter (or pair of letters) found in the periodic table of elements. For instance, the X might represent hydrogen (H) or helium (He).
The numbers off to the side refer to the number of protons and neutrons.
The larger number up top is the protons + neutron count.
The smaller number is the number of protons only.
For example, writing [tex]^{14}_6X[/tex] means there are 6 protons + 8 neutrons = 14 total.
The proton number is the key indicator of what element we're dealing with. If we want the elements to be the same, they need to have the same bottom number. Choices A, C, and D are ruled out because of this.
Choice B is the answer because the 6 at the bottom tells us we're dealing with carbon with 6 protons. Carbon-12 has 6 protons + 6 neutrons to get 6+6 = 12 items total. So the naming of "carbon-12" helps us figure out which isotope of carbon we're dealing with. The abundance of carbon-12 is somewhere around 98.84% and 99.04%
In slight contrast, carbon-13 has 1 extra neutron compared to carbon-12 (6 protons + 7 neutrons = 13 total). The abundance of carbon 13 is roughly between 0.96% and 1.16%.
Even more rare is carbon-14. The percentage value is so very very small that it's practically 0% (though this isotope is around in small trace forms).
Side note: carbon-11 is also an isotope of carbon, but it's not found in nature. Rather, it's manmade or synthetic.
