Germanium and silicon behave as semiconductors because

The correct answer is Option (2) → the energy gap between their conduction and valence bands is of the order of 1 eV

A material is classed by its energy gap between the valence band and the conduction band. A gap of about 1 eV is small enough for some electrons to be lifted across by thermal energy, but large enough that the conductivity stays far below that of a metal; this is what makes a semiconductor. The gap is about 1.1 eV for silicon and 0.7 eV for germanium. In a metal the bands overlap, and in an insulator the gap is large.

NCERT (2026-27):
Extrinsic Semiconductor – “… The difference in the resistivity of C, Si and Ge depends upon the energy gap between their conduction and valence bands. For C (diamond), Si and Ge, the energy gaps are 5.4 eV, 1.1 eV and 0.7 eV, respectively. Sn also is a group IV element but it is a metal because the energy gap in its case is 0 eV.”