in a fusion reaction, two (blank) atoms fuse to produce a helium atom.?

hydrogen...

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actually, the reaction is more like this...

2 H -----> 1 D + 1 e(+) + 1 neutrino

1 H(+) + 1 D -----> 1 ³He + 1 gamma ray

2 ³He -----> 1 ⁴He + 2 H(+)

the D is deuterium and the e(+) is a positron.

So I suppose you could say the overall reaction is 4 H's fuse together to form 1 helium + 2 gamma rays + 2 neutrinos + 2 positrons + energy....

Deuterium atoms (Heavy Hydrogen) which also have a Neutron in the Nucleus and ONLY Deuterium will fuse into Helium. (A Helium atom has 2 Neutrons, 2 Protons and 2 electrons. Normal Hydrogen only has 1 of each).

No. There is practically no "heavy hydrogen" in the sun.

And No. The reaction is NOT between two hydrogens which fuse to make helium. Where are the two neutrons going to come from? No. Your question is incorrect.

The reaction is between four hydrogen atoms and two electrons which produce a helium nucleus (2 protons and 2 neutrons) and 2 neutrinos.

The overall reaction is:

4(1H1) + 2(-1e0)--> 1(2He4) + 2 neutrinos

Heavy Hydrogen (Deuterium) atoms.

Hydrogen.

This is what occurs in stars such as our sun.

hydrogen