In which of these processes platinum is used as a catalyst ? 

The Ostwald process is used for the manufacturing of nitric acid. During this process, platinum is used as a catalyst to convert ammonia into nitric oxide which is further converted into nitric acid.

The reaction is as follows:

$4{\mathrm{NH}}_3+5{\mathrm{O}}_2\stackrel{\text{ (in presence of Pt) }}{\to }4\mathrm{NO}+6{\mathrm{H}}_2\mathrm{O}$

Hence, option (A) is correct.