Which of the following is true of subatomic particles in a nucleus of vanadium-50? a. a. it has 23 protons and 28 neutrons. b. it has 27 protons and 23 neutrons. c. it has 27 protons and 51 neutrons. d. it has 23 protons and 27 neutrons.

From the image provided, we can see that the atomic number of Vanadium is 23. Atomic number = number of protons. It is like an ID card number for the element and no other elements have the same proton / atomic number. Therefore, we can already eliminate B and C because their proton number is not correct. 

Vanadium-50 is an isotope of Vanadium. the '50' means that the sum of protons and neutrons in the isotope equals to 50. We can calculate the number of neutrons by subtracting 23 from 50. Therefore, we know that the number of neutrons is 27.

So, D is the correct answer.