What does decrease in Stock mean? Change in Stock or depreciation ? I thought it would be change in Stock but in that case it wouldn't have been dedcuted in this question

Dear student,

Decrease in stock and increase in stock are part of change in stock.
Change in stock = ( Opening stock - Closing stock)
If change is stock is positive, i.e. opening stock is more than closing stock, it is called increase in stock. 
If change in stock is negative, i.e. closing stock is more than opening stock, it is called decrease in stock.
In given question decrease in stock is deducted because it is a negative figure.

Regards