So here this example question states which the following isotopes will lie to the left of the neutron to proton curve. Here we have Zirconium 90, Thorium 230, Palladium 110 and Mercury 200.
So here Zirconium 90 its mass number is 90. Looking on the periodic table its atomic number is 40. This would mean that it has 40 protons and then 90 - 40 is 50. Neutrons here are atomic number is the number of protons. So we have 40 protons and then we just trace up to 50 which is around here. We can see that that's not to the left of our neutron to proton curve which is our band of stability. So this is out.
Thorium 230. So we have 230 for the mass number for thorium. Its atomic number is 90. We have 90 protons and then we're going to say here 230 - 90 is 140, so we have 90 and we just take it all the way up to 140, which is kind of off scale. So again, it's not to the left.
Palladium 110. Palladium has an atomic number 46. So we have 46 protons and then here we're going to do 110 - 46 here. So when we do that, we're going to get 64. So then here we have, let's see 46 which is around here for protons and we just go up to about 64, which is going to put us about here. So we're just slightly to the left. So this seems to be our answer, but let's do the last one.
Mercury 200. So 200 for Mercury and its atomic number is 80. So we have 80 protons and we have 120 neutrons, so 80 and then we go up to 120. So we're going to be to the right of the curve. So option D does not work here. Only option C is the correct answer.