Answer:
The unknown substance is Au ( gold)
Explanation:
SiO2 has a high melting point, is insoluble in water but is a very poor conductor. So the statements do not apply here.
F2 is soluble in water, has a melting point way lower than 1064 °C, and isa gas. F2 doesn't apply here.
Au is insoluble in water, has a high melting point, is a good conductor and is hard. Gold applies are all those statements.
Kbr is good soluble in water, and is a bad conductorbecause it is just a powder, and the molecules have no mobility, and therefore it cannot conduct electricity. The unknown substance is not Kbr.
The unknown substance is Au ( gold)