Magnesium has atomic no 12
Electronic configuration given by
Or
Answer to this is Radioactive isotopes.
Isotopes are the species of the same element having different atomic masses that means the number of protons remains the same but number of neutrons do differ. For example
and
are the two isotopes of Hydrogen (
).
Radioactive isotopes are the isotopes which release some kind of energy in the form of alpha particles, beta particles or gamma radiation. Examples of each of the decay processes are :
Alpha Decay: In this decay one alpha particle having atomic mass 4 and atomic number 2 or we can say a He molecule will come out. 
Beta Decay: In this decay a
particle is emitted increasing the atomic number of the reactant by 1 unit.

Gamma Radiation: In this type of reaction only radiation is emitted out which does not change the original molecule.

Answer: Out of the given options acetic acid is in household vinegar.
Explanation:
Household vinegar is the one that is commonly used in our home while cooking a number of dishes.
The common name of vinegar is acetic acid and its chemical formula is
.
For example, vinegar (acetic acid) is used while making noodles.
Thus, we can conclude that out of the given options acetic acid is in household vinegar.
Answer:
A, B, and C
Explanation:
Atoms do in fact have mass! They are also the basic unit of a chemical element. Matter is in fact made of atoms, but atoms do not take up space.
Atoms are filled with empty space, excluding it out from the rest of the answers.
So yes, A, B, and C should be the correct answers