The Quantum Phenomenon Known as the "Tunnel Effect" Refers to $\alpha$

The quantum phenomenon known as the "tunnel effect" refers to

A) highly eccentric electron orbits penetrating inner closed shells.
B) the fine structure exhibited by many spectral lines.
C) the small but finite probability that an $\alpha$ -particle originally within the nucleus will be found outside the nucleus.
D) the penetration of shielding by high-energy fission neutrons.
E) an orbital electron penetrating the nucleus and undergoing electron capture.