Even if historians can't know the exact reasons for a business's failure, it doesn't effect whether they tend to study successful businesses more or overestimate past business success without considering both successes and failures.

We don’t know if business historians are studying "first-person accounts of business strategies." Even if they are, (B) doesn’t explain whether using present-day cultural assumptions affects how accurately historians assess past business successes.

Irrelevant— we don’t know whether the historians are studying public documents, internal documents, or both. We also don’t know how, if at all, any of these documents might affect historians’ ability to accurately assess past business successes.

This strengthens the argument because if historians can’t access records from bankrupt businesses, they can’t accurately account for business failures or judge how often they overestimate past business successes. As a result, they are likely to overestimate those successes.

Irrelevant— the fact that the historians are trained in business administration tells us nothing about whether they’re more or less likely to overestimate past business successes.