What is an alpha particle? a particle that has the same mass as an electron but the opposite charge. a very fast, high-energy electron. a helium nucleus without its electron cloud. a subatomic particle created by bombardment of a positron with a neutron. pure energy of a certain wavelength.

A helium nucleus without its electron cloud.

Explanation:

An alpha particle is composed by two protons and two neutrons. The most common isotope of helium, with an abundance of 99.9998%, Helium-4 has a nucleus composed of two protons and two neutrons. Of course, the helium also has electrons. For this reasons its often say that the alpha particle its an Helium Nucleus, or, as stated in the option, an helium nucleus without its electron cloud.

the answer is 100. i just did this lesson and got a 100% on it. : )

-fishtank