What is a point defect?

The correct statement$(s)$ regarding defects in solids is/are:

In the $AgBr$ crystal,the size of ions follows the order $Ag^{+} << Br^{-}$. Which of the following characteristics is shown by $AgBr$?

The formula of a metal oxide with a metal deficiency defect in its crystal is $A_{0.8}O$. The crystal contains $A^{2+}$ and $A^{3+}$ ions. The fraction of metal existing as $A^{2+}$ ions in the crystal is -

The color of metal ions in flame is due to which of the following?

At what temperature will the Schottky defect be maximum?