a. Earth's orbit would not change.
Explanation:
The force that keeps the Earth in circular motion around the Sun is the gravitational force between the Earth and the Sun, whose magnitude is given by:
[tex]F=G\frac{m M}{r^2}[/tex]
where
[tex]G=6.67\cdot 10^{-11} m^3 kg^{-1}s^{-2}[/tex] is the gravitational constant
m is the mass of the Earth
M is the mass of the Sun
r is the distance between them
From the equation, we observe that this force depends only on three factors:
If the Sun is replaced by a black hole of the same mass, none of these factors would change (the central mass M would remain the same), therefore the gravitational force F would not change, and therefore the orbit of the Earth would not change.
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