When two nouns connected by an 'and' act as a single unit, they are treated as singular subject and take singular verb.

Ham and eggs is a high cholesterol breakfast.
Hams and eggs are high cholesterol breakfasts.

"and" is the only additive phrase which which changes the two singular subjects to plural. Hence and is followed by are. a can be used to specify the breakfast which is singular. My take would be on option B)

I accept that "hams and eggs" can be combined into a single breakfast meal.
In that case, one could refer to such menu as a singular breakfast meal.
Another example in this category:
Drinking and driving is a dangerous habit.

On the other hand, the second example treats the subjects as individual components:
Hams is a high cholesterol breakfast; also,
Eggs are high cholesterol breakfasts
As a result, we say, "hams and eggs are high cholesterol breakfasts.

My question:
Is it a convention on the GMAT to simply use PLURAL verb on instance of "AND" or do we
need to consider the context before making such decision as shown in the examples above?

Looking forward to your contributions.