a) This seems a self planned fire at the restaurant. Any person has put some gasoline in restaurant and while doing so some gasoline may fallen on him/her and then he set fire in the restaurant.
Later when fire is ceased , that person cleaned his/her hand with that cloth and put it on window sill , that is why cloth was unburnt.
TUTVUMU VIVIETU TUNT 2. You've been called to investigate the scene of a very large restaurant...