The correct answer is Rare.
In linguistic analysis, vocabulary acquisition relies heavily on identifying precise semantic equivalents. The adjective infrequent is defined as something that happens at long intervals, is not habitual, or occurs uncommon times. The term rare serves as its absolute synonym, meaning exceptionally uncommon or seldom occurring. Both words accurately quantify a low frequency of occurrence within a given period, making them completely interchangeable in formal and academic English contexts.
The remaining distractor options fail to provide an exact semantic match due to incorrect frequency scaling. For example, never denotes a complete, absolute absence of occurrence, which is too extreme. Conversely, usual represents the polar opposite, indicating a high-frequency habit or standard norm. While sometimes indicates an intermittent occurrence, it implies a higher, more regular frequency than the scarcity connoted by infrequent. Therefore, rare is the only option that precisely captures the specific low-frequency essence of the target word.