Question

$U-235$ requires a neutron to be bombarded into it to undergo fission because

• A. The energy of U-235 increases due to collision with the incoming neutron
• B. The U-235 nucleus absorbs the neutron and becomes U-236 which is highly unstable
• C. The neutron due to its high energy breaks apart the nucleus
• D. None of the above.

Answer 1

The correct option is B

The U-235 nucleus absorbs the neutron and becomes U-236 which is highly unstable

First of all, don't try to imagine the scenario from the classical picture like a neutron hitting the nucleus and breaking it apart like a bullet would do to a trick.
U-235 isotope is stable. It doesn't undergo fission. Another isotope of Uranium U-236 is found to be highly unstable which doesn't even exist because as soon as it is formed it fissions.
So, what a neutron basically does is that it is absorbed by a U-235 and U-235 becomes its unstable isotope U-236.
${}^{235}U+{}_0^{1}n\to {}^{236}U$.
And thus U-236 fissions eventually.