Answer:
a. The orbit of Earth around the Sun becomes smaller.
Step-by-step explanation:
Now, it in the given question it is stated that what will happen when earth's velocity gets doubled. Therefore, their will be no change in quantities like mass of sun and earth.
Now, we know that orbital velocity of earth around sun is given by,

Where G is gravitational constant and
is the mass of sun and
R is radius of orbit of earth around sun.
Now, let the velocity get doubled and let it be
.
Then,
.
Here, r is radius of orbit of earth around sun when earth velocity gets doubled.
Now,
.
After all calculations we get,
.
Therefore, The orbit of Earth around the Sun becomes smaller.
Hence, a. is correct.