Which factor most commonly indicates a bull market?

A bull market is characterized by significant increases in stock prices, typically accompanied by strong investor confidence and optimism regarding the economy's growth potential. This scenario is represented by the option focusing on high investor optimism and rising stock prices. During a bull market, investors generally expect continued price increases and are more willing to buy, which further drives up demand and prices.

The essence of a bull market is the prevailing positive sentiment that leads to accumulation and investment in equities, creating an upward momentum in the markets. This environment often encourages more capital inflow as investors feel secure in their investments, anticipating further gains.

The other options reflect conditions that are typically associated with negative market environments, such as price drops, increased regulatory uncertainty, and bankruptcies, none of which foster the optimism necessary for a bull market. Thus, the indication of rising stock prices alongside robust investor optimism defines the bullish sentiment and outlines the correct answer.