Which of the following can a level hierarchy do?

A level hierarchy in SAP HANA is primarily designed to allow users to organize data into structured levels, which can be particularly useful for analytical processes and reporting. The feature that enables drilldown functionality in a value help list allows users to further explore data by drilling down into the hierarchical structure. This capability enhances the user experience by providing deeper insight into data relationships and hierarchies, facilitating better decision-making.

In the context of other options:

While time-dependency allows for the representation of data changes over time, this is typically associated with attributes or characteristics rather than being a fundamental feature of a level hierarchy. Similarly, a bill of materials (BOM) structure is more representative of a product's components and not the primary functionality of a hierarchical structure. The concept of variable deepness in data, while interesting, is more related to how data is displayed or aggregated rather than a characteristic of level hierarchies themselves.

Thus, the ability to enable drilldown in a value help list stands out as a key function specifically provided by level hierarchies in SAP HANA, as it directly enhances data exploration capabilities.