An element X belongs to the 3rd period and 15th group of the modern periodic table.​ What is the atomic number of element X?

The correct option is A 15
The element X belongs to the 3rd period and 15th group. So, the number of shells present in the atom will be three, and five electrons will be present in its valence shell. Therefore, the electronic configuration will be 2, 8, 5. And, atomic number of X = 2 + 8+ 5 = 15.