The reaction which is used in black and white photography is:

• A
  $2 KClO_{3}(s) \xrightarrow{\text{Heat}} 2 KCl(s) + 3 O_{2}(g)$
• B
  $CH_{4}(g) + Cl_{2}(g) \xrightarrow{\text{Sunlight}} CH_{3}Cl(g) + HCl(g)$
• C
  $2 NaCl(s) \xrightarrow{\text{Electric current}} 2 Na(s) + Cl_{2}(g)$
• D
  $2 AgBr(s) \xrightarrow{\text{Sunlight}} 2 Ag(s) + Br_{2}(g)$