These are ingredients for a stuffing recipe. which ingredient is necessary to prevent the stuffing from burning?
cubed bread crumbs, a. melted butter, onion, celery, b. mushrooms, c. egg, chicken broth d. salt, pepper.

The necessary ingredients to prevent the filling from burning are:

C) Egg, chicken broth.

Explanation:

The egg in any filling creates a layer on the dough when adding heat to the dough the egg is heated first causing the dough to become crispy and preventing the filling from burning. Something like a protective layer.

And the chicken broth causes the filling to become moist, this in order to prevent heat from burning the filling before the dough is at its point, since the heat is kept inside the dough and can burn the filling first.