I believe is A) Inner core
The force of gravity between the astronauts is 
Explanation:
The magnitude of the gravitational force between two objects is given by:
where
:
is the gravitational constant
are the masses of the two objects
r is the separation between them
In this problem, we have two astronauts, whose masses are:

While the separation between the astronauts is
r = 2 m
Substituting into the equation, we can find the gravitational force between the two astronauts:

Learn more about gravitational force:
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What is the SI (metric) unit of FORCE?
with symbol ( N )
All the best !
A) We balance the masses: 4(1.00728) vs 4.0015 + 2(0.00055)4.02912 vs. 4.0026This shows a "reduced mass" of 4.02912 - 4.0026 = 0.02652 amu. This is also equivalent to 0.02652/6.02E23 = 4.41E-26 g = 4.41E-29 kg.
b) Using E = mc^2, where c is the speed of light, multiplying 4.41E-29 kg by (3E8 m/s)^2 gives 3.96E-12 J of energy.
c) Since in the original equation, there is only 1 helium atom, we multiply the energy result in b) by 9.21E19 to get 3.65E8 J of energy, or 365 MJ of energy.
F = G*((m sub 1*m sub 2)/r^2)