A magician during a show makes a glass lens μ = 1.5 disappear in a trough of liquid. The refractive index of the liquid would be:

1.5

The correct answer is Option (1) → 1.5

for the glass lens to "disappear" in the liquid, the refractive index of the liquid must match the refractive index of glass.

$μ_{liquid}=μ_{glass}=1.5$

This is because, when the refractive indices are the same, no refraction occurs at the boundary, and the lens become invisible.