The isotope carbon-14 has 6 protons and an atomic mass of 14. Carbon-14 has:
14 neutrons
8 neutrons
20 neutrons
6 neutrons

The isotope carbon-14 has 6 protons and an atomic mass of 14. Carbon-14 has 8 neutrons.

Explanation:

An atom consist of electron, protons and neutrons. Protons and neutrons are present with in nucleus while the electrons are present out side the nucleus.

All these three subatomic particles construct an atom. A neutral atom have equal number of proton and electron. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other. For example if neutral atom has 6 protons than it must have 6 electrons. The sum of neutrons and protons is the mass number of an atom while the number of protons are number of electrons is the atomic number of an atom.

In given atoms ¹⁴₆C  atomic mass of carbon is 14 . it means Carbon has 6 protons and 8 neutrons in its nucleus because

In C:

Number of neutrons + protons = atomic mass

8 + 6 = 14 amu (mass number)

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

it means there are 3 atoms of oxygen