Explanation
Meaning of the sentence: The sentence implies that all those people who are willing to honestly assess today's era are also part of an important British tradition, which many have forgotten.
Diction: Whom is an objective pronoun that is used when you intend to refer to the object of a verb or preposition. To decide whether the sentence requires who or whom, flip the sentence into a question. Now, answer the question. If the answer is he or she, you require who. If the answer is him or her, you require whom. For example, consider the following sentence:
Q: Who/whom did she marry?
A: She married him.
Since the answer is him, you require the objective pronoun whom.
Flipping the given option into a question, we get the following sentence:
Q: Who/whom is willing to undertake…
A: He/she is willing to undertake…
Since our answer is he or she, the correct pronoun is who.
Options A, C and E use the incorrect pronoun whom, so they can be ruled out.
Logical Prediction: The sentence makes a reference to a British tradition and describes it as one that has been almost forgotten. To emphasize the point, the phrase if not largely forgotten, has been pushed to the margins has been used. Although and though do not bring out the contrast that is intended in the sentence. Similarly, in spite of not being largely forgotten, doesn't fit in logically with the intended meaning and is wordy.
Options A, D and E contain this error.
Idioms: Marginalized, as used in option C, is incorrect to be used to describe a tradition. The word is usually used to describe people, groups or concepts that are treated as insignificant or peripheral. Take over, as used in option D, has a completely different meaning (assuming control over something). Undertake is the correct verb here.