Answer is the threat of substitute products or services.
Rivalry among the existing competitors was not a significant factor in the BlackBerry's downfall because they were not able to produce any form of alternative to the iPhone, and the users switched from the BlackBerry phones to the iPhones. The danger of replacement products is one of the Porter's five factors, and it indicates that there are alternative items that are likely to steal the company's market share. The downfall of the BlackBerry was caused by the substitute product in the shape of the iPhone.
All of the other allegations are untrue because there were no new entrants and the suppliers threatened because they were unable to upgrade their product.
Therefore, (B) Threat of substitute products or services is the correct answer is the correct answer.
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