ANS:D - Large weight of liquid retained.

An undesirable property in a tower packing is large weight of liquid retained. Tower packings are materials or structures used in packed columns to provide a large surface area for mass transfer between gas and liquid phases. The effectiveness of tower packings depends on several factors, including their surface area, void fraction (free cross-section), and weight. While large surface area per unit volume and large free cross-section are desirable properties as they promote efficient mass transfer, low weight per unit volume is also advantageous as it reduces the load on the column structure. However, a large weight of liquid retained by the packing material is undesirable. Excessive liquid holdup can lead to flooding and reduced efficiency of the packed column. It can also increase the pressure drop across the column, impacting its performance. Therefore, among the options provided, large weight of liquid retained is the undesirable property in a tower packing.