Final answer:
A pullback in a bull trend fails when it breaks a previous low, indicating a potential trend reversal and weakening of the current upward trend.
Step-by-step explanation:
When discussing a pullback in a bull trend, it typically fails when it breaks a previous low. This indicates a potential trend reversal or weakening of the current trend. A pullback is considered a temporary decrease in the price of an asset within an ongoing uptrend. When the pullback goes below a significant previous low, it can signal a change in market sentiment, potentially leading to the end of the bullish trend.
Therefore, option B) When it breaks a previous low is the correct answer. This is different from reaching a new high, which would actually confirm the continuation of the bull trend. Increased volume can provide further confirmation of the trend's direction, and lack of follow-through may simply mean a pause in the uptrend, not necessarily its failure.