The correct answer is option 3: 143.

Uranium-235 is an isotope of uranium, which means it has a different number of neutrons compared to the most common form of uranium. The number 235 in the name refers to the total number of protons and neutrons in the nucleus.

To determine the number of neutrons, we need to subtract the atomic number (which represents the number of protons) from the total mass number. In the case of Uranium-235, the atomic number is 92 (option 2), and the total mass number is 235.

Thus, the number of neutrons can be calculated by subtracting 92 from 235:

Neutrons = 235 - 92 = 143.

Therefore, the correct answer is option 3: 143.

The other options, 235 (option 1), 92 (option 2), and 51 (option 4), are incorrect as they do not represent the number of neutrons in the nucleus of Uranium-235.