Assertion :Internal energy of a system is an extensive property.
Reason :The internal energy of a system depends upon the amount and physical state of the substance.

As internal energy is $U=m{c}_{v}dT$ so it depends upon mass $m$, and specific heat $c_v$, i.e., it depends upon amount and state of the substance.

So Internal energy is an extensive property of system.

For example if we take a container filled with gas and partition it into two halves, mass in each side also get halved, thus internal energy $U=m{c}_{v}dT$ is also reduced to half.

Thus, both assertion and reason are true and reason is correct explanation of the assertion.