Answer
$93.0\text{K}$
$23.15\text{K}$
$126.98\text{K}$
Work Step by Step
We will convert all the critical temperatures (Tc) to the Kelvin scale, which is standard for scientific comparisons.
$K=C+273.15$
$YBa_2Cu_3O_7$:
$93\text{K}$
$Nb_3Ge$:
$-250^\circ\text{C}+273.15=23.15\text{K}$
$HgBa_2CaCu_2O_6$:
$C=\frac{5}{9}(-231.1^\circ \text{F}-32)\approx -146.17^\circ\text{C}$
$K=-146.17^\circ\text{C}+273.15=126.98\text{K}$
$HgBa_2CaCu_2O_6$ has the highest superconducting temperature at $126.98\text{K}$
