explain why most ecologists would agree to the term "a forest ecosystem is more suitable than a cornfield"?

I assume you mean more suitable for supporting wildlife - a forest system contains a range of different species of plants, insects, birds etc.  The different habitats provide different species with suitable enviroments in which to survive.

A cornfield is usually a monoculture - that is it contains only one type of plant species.  It does not provide habitat for bird species and limited habitat for animal species.  The lack of varied habitats and food sources reduces the biodiversity - i.e the number of different species living there.  High biodiversity is important for the survival of an ecosystem, as it can respond better to periods of adverse conditions (eg. drought, cold, etc.).

Also, a cornfield is harvested every year, which is disruptive to any species living there.

The answer to the second question is B.