Here it says select the correct statement by alpha decay. Here the parent nuclide gains 2 protons and two neutrons. No, one doesn't gain them and loses them. To create the alpha particle produces radiation by absorbing A helium nucleus. No, it doesn't absorb the helium nucleus. It emits or gives off or loses the helium nucleus. That's what the alpha particle is.
Alpha particles can easily penetrate cells well. In terms of size, the alpha particles are the largest, so it's hard for them to penetrate our skin and therefore penetrate ourselves as easily when alpha part. When alpha decay occurs in an unstable atom, the mass number decreases by 4 and the atomic number decreases by two. This is true because what are we losing? We're losing 2 protons and two neutrons.
2 protons being lost would lead to a decrease of our atomic number by two, and losing 2 protons and two neutrons would give us a mass number of four. So your mass number would decrease by 4. So here option D is a true statement.