The metalloids, elementes with properties in between metals and no metals are: boron, B; silicon, Si; germanium, Ge; arsenic, As; antimonium, Sb; tellurium, Te; polonium, Po; and astatine, At.
Germaniun and arsenic belong the the fourth period.
Germanium is an important semiconductor, very used in transistors and integrated circuits, while arsenic has different uses (it is not a semiconductor).
Therefore, the answer is germanium.
