Question

What feature of double-glazed windows makes them good insulators? a. Thick glass is highly viscous. c. Heat conduction cannot take place through solid glass b. There is a vacuum between the panes of glass d. Thermal energy is not transferred through the special chemical glazing.

Answer 1

None  of the above.
The air gap between the panes adds some thermal   insulation.
About 1/2"  is best.  Less than that  you  get     conduction.  More than that you get more convection between panes.   Double glazed windows do not usually have a vacuum in them --   there could be argon  tho...  

Answer 2

Many double-pane glass windows have a partial vacuum between the plates.  This is a so-called VIG or Vacuum Insulated Glass.  This prevents convection from transferring heat (A google search of VIG will reveal this).   Other times a less convective gas (noble gasses) are used since they don't carry heat in rotational modes like other gasses.  In vacuum panes, the main transport of heat is through conduction in the glazing.