Question

out of two solar cookers one was covered with a plane glass slab and other was left open which of the two solar cooker will be more efficient and why

Answer 1

the one which is covered with glass slab will be more efficient because the glass slab do not allow the heat to escape and the heat gets trapped increasing the temperature

Answer 2

For it to be a complete solar cooker actually it needs glass covering on the top to create an a Greenhouse effect so you really need a glass over it and you can conclude that the one which is having is more efficient.