Which best describes nuclear fission?
a. a nucleus spontaneously splits and absorbs energy.
b. two nuclei spontaneously combine and absorb energy.
c. a nucleus collides with a neutron and splits, releasing energy.
d. nuclei combine to form a heavier nucleus, releasing energy.

the answer is c

Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes.Most fissions are binary fissions (producing two charged fragments), but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced, in a ternary fission

The answer should be c.

Yes it would be a physical change.