To determine the appropriate degree of comparison for the blank in the sentence, we should analyze the options provided and understand their usage:

• most magnificent - Superlative degree; used to compare one thing against all others in the same category.
• magnificently - Adverb form; modifies verbs to express manner, not applicable in this context.
• magnificent - Positive degree; describes a noun without comparing it to others.
• more magnificent - Comparative degree; compares one thing to another of the same kind.

The sentence is: "This painting is undoubtedly ______ example of post-modern expressionism to emerge in the late 20th century." The context of the sentence is to indicate the highest degree of quality, as it suggests uniqueness or supremacy in the category of post-modern expressionism paintings that emerged in the late 20th century.

Therefore, the superlative form most magnificent is appropriate here, as it implies that this painting stands out among all others of the same kind.

Let's analyze why other options are not suitable:

• magnificently is an adverb, so it cannot fill the blank that requires an adjective.
• magnificent is in the positive degree, not highlighting a comparison.
• more magnificent is the comparative degree, suitable when comparing two specific entities, but not ideal for expressing singularity out of a group.

In conclusion, the correct choice is most\ magnificent because it appropriately signifies that this painting is the pinnacle example among others in its category.