Which adverb describes a lack of frequency?

The adverb "rarely" appropriately describes a lack of frequency because it indicates that something happens infrequently or almost never. It conveys a sense of scarcity in occurrence, suggesting that the action or event does not happen often at all. This aligns directly with the concept of describing something that lacks frequency.

In contrast, the other adverbs imply varying levels of frequency. "Always" indicates that something occurs all the time, while "often" suggests a regular or frequent occurrence. "Seldom" also suggests infrequency but is typically understood as occurring less than "often," whereas "rarely" emphasizes an even stronger lack of frequency. Therefore, "rarely" is the best choice to convey the idea of something happening infrequently.