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

Identify the correct statement about the crystal defects in solids.

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 -

Which of the following point defects are shown by $AgBr_{(s)}$ crystals?
$A$. Schottky defect
$B$. Frenkel defect
$C$. Metal excess defect
$D$. Metal deficiency defect

Explain the metal excess defect.

The appearance of colour in solid alkali metal halides is generally due to