Question

Positronium is a name given to an atom-like combination formed between _____.
A. a positron and a proton
B. a positron and a neutron
C. a positron and an $\alpha$-particle
D. a positron and an electron

Answer 1

The answer to this lies behind in the name itself, Positronium, a quantum state in which a particle called positron is bound to some other particle in the atomic structure with some forces of attraction that is binding the two particles with each other.

Complete step by step answer:
Positronium is a system which consists of an electron and its anti-particle positron bound together into an exotic atom, which is very unstable. The energy levels of electrons and positrons are similar to that of hydrogen atoms.

Let us discuss about positron:

-A positron is a particle with the same mass as that of an electron but with an opposite charge. So, positrons are called antiparticles of electrons.

-When a positron encounters an electron, the two completely destroy to yield energy. It is different from electrons as it has positive charge. Positrons are formed during decay of nuclides and have an excess of protons in their nucleus as compared to the number of neutrons. When decay takes place, these radionuclides emit a positron and a neutrino.

- The mass of a positron is equal to the mass of an electron. The value of the mass of the positron is 9.11 x ${{10}^{-31}}$ kg.

- The positron does not exist in nature. The system is unstable as the particles bound produce 2 or 3 gamma rays, depending on the relative spin states. After a few fractions of billionth of a second, the state decays out into gamma rays.

We are familiar with electrons. Electrons are particles with the smallest electrical charge. The charge is equal in magnitude but opposite in sign with that of proton. The mass of the electron is 9.11 x ${{10}^{-31}}$ kg. Electrons form bounding with positron in a system called positron.
Positronium is a name given to an atom-like combination formed between a positron and an electron.
Hence, the correct answer is option D.

Note:
The positron need not be confused with proton, as the name sounds similar. But, there exist differences between proton and positron.

PROTON	POSITRON
Proton is a particle of normal matter.	Positron is a particle of antimatter, which does not exist in real life.
Mass of the proton is $1.6\times {{10}^{-27}}$ kg.	Mass of the positron is 9.11 x ${{10}^{-31}}$ kg.
Protons are very stable particles in normal conditions.	Positron is unstable under normal conditions.