The correct option is D violet
The white moist potassium iodide paper turns brown as soon as it comes in contact with nitrogen dioxide gas.
However, if it is kept in contact for a long time, they react with each to form iodine gas which results in a violet tinge on the paper.
The equation for the same can be written as:
2NO2(g)+2KI(aq)→2KNO2(aq)+I2(g)