ANS:C - Packed column (stacked)

Among the options provided, the column that typically gives the lowest pressure drop per unit height for the same diameter is the sieve-plate column. Here's why:

1. Bubble-Cap Column: Bubble-cap columns have relatively higher pressure drops per unit height compared to sieve-plate columns. This is because bubble caps introduce additional resistance to gas flow as the gas bubbles must pass through the caps.
2. Packed Column (Stacked): Packed columns with stacked packing materials, such as structured packing or random packing, typically have higher pressure drops per unit height compared to sieve-plate columns. The tortuous paths the gas and liquid must follow through the packing material create significant resistance to flow.
3. Randomly Packed Column: Randomly packed columns also tend to have higher pressure drops per unit height compared to sieve-plate columns. The irregular arrangement of the packing material leads to turbulence and increased resistance to flow.
4. Sieve-Plate Column: Sieve-plate columns, also known as perforated plate columns, use perforated plates with holes or sieve trays to facilitate gas-liquid contact. The design of sieve trays allows for efficient mass transfer while minimizing pressure drop. The open area of the perforated plates reduces resistance to gas flow, resulting in lower pressure drops per unit height compared to other column types.