Final answer:
The isotope with 39 protons, 39 electrons, and 30 neutrons is Yttrium-69 (D). The mass number is the sum of the protons and neutrons, which equals 69 for this isotope.
Step-by-step explanation:
To identify an isotope given the number of protons, electrons, and neutrons, you need to first understand that the number of protons determines the element and the atom's identity. As such, with 39 protons, the element is yttrium (Y), because yttrium has an atomic number of 39. The number of neutrons can vary among isotopes, which provides different mass numbers for the element. The mass number is the sum of the protons and neutrons in the nucleus. So for an isotope with 39 protons and 30 neutrons, the mass number would be 39 + 30 = 69. Therefore, the isotope is yttrium-69, because the mass number is indicated by the number following the name of the element.
Given the options, the correct response is Yttrium-69 (D). The symbol for this isotope would be written as 69Y39, where the superscript represents the mass number and the subscript represents the atomic number.