Final Answer:
D: Lithium (Li) is the substance that when combined with germanium will cause the germanium to emit free electrons.
Step-by-step explanation:
Lithium, with its electron configuration of 1s²2s¹, has an extra valence electron that it can readily donate to germanium, which has an electron configuration of 1s²2s²2p²3s²3p²3d¹⁰4s². This donation of an electron from lithium to germanium results in the formation of a negatively charged germanium ion (Ge⁻) and a positively charged lithium ion (Li⁺). The free electron that is emitted during this process can contribute to the conductivity of the germanium material.
The other options, oxygen (O2), fluorine (F2), and phosphorus (P4), are not as likely to donate electrons to germanium. Oxygen and fluorine are electronegative elements that tend to attract electrons, while phosphorus is a semiconductor with its own valence electrons that it can share.
Therefore, the combination of lithium and germanium is the most likely to cause the germanium to emit free electrons and become an n-type semiconductor.
Option D is answer.