Answer:
A. they are potentially devastating for individual investors, whose
Step-by-step explanation:
B. they can potentially devastate individual investors in that their - Meaning Issue. Initial sentence bear markets are devasting individual investors it doesn't talks about capability. Option B refers to capability.
C. for individual investors they are potentially devastating, since their Meaning Issue - In Original sentence "whose" describes institutional investors and we dont know whether nest eggs is reason for this.
In D and E pronoun-antecedent error. It cannot refer to plural Bear Markets.