63ʳ B.P.S.C. (Pre) 2017
Quartz crystals used in clocks are :
aSodium silicate
bSilicon dioxide✓ Correct
cGermanium oxide
dTitanium dioxide
eNone of the above/More than one of the above
Explanation
Quartz crystals used in watches are silicon dioxide ($SiO_2$). In nature, some forms are found as rock crystal and rhinestone. However, those used commercially are generally man-made. Quartz crystals are also used in radio and television.
general-science