A mnemonic device is a:

A mnemonic device is indeed a memory aid designed to help individuals improve their ability to remember information. These techniques often involve associating new information with familiar concepts, creating visual images, or using acronyms and rhymes to enhance recall. The use of mnemonic devices is effective because it leverages existing memory structures, making it easier to retrieve new information.

In contrast to the other options, sensory memory refers to the initial stage of memory that holds sensory information for a brief period, while a measure of memory pertains to the evaluation techniques used to assess memory performance. Automatic processing refers to the unconscious encoding of information without deliberate effort, which is different from the intentional strategies involved in using mnemonics. Therefore, identifying a mnemonic device specifically as a memory aid accurately captures its primary function in the realm of psychological tools for enhancing memory retention and retrieval.