ANS:B - Bubble

When a high liquid hold-up is required in a reactor for gas-liquid reactions, a bubble column is preferred. In a bubble column reactor, gas is introduced at the bottom of the column, forming bubbles that rise through a liquid phase. These bubbles create a large interfacial area between the gas and liquid phases, facilitating mass transfer and reaction between the reactants. The liquid phase is held up by the bubbles, allowing for a high residence time and increased contact between the phases. The design of bubble column reactors allows for a high liquid hold-up, which is beneficial for gas-liquid reactions requiring longer residence times or higher conversions. Additionally, bubble column reactors are relatively simple in design and operation, making them suitable for a wide range of gas-liquid reaction systems. Therefore, a bubble column is preferred when a high liquid hold-up is required in a reactor for gas-liquid reactions.