What is a key feature of the KYX-15?

The key feature of the KYX-15 being related to its use for filling cryptographic devices highlights its significance in secure communications. This involves programming or updating encryption keys in hardware devices that require cryptographic protection. The KYX-15 plays an essential role in ensuring the integrity and confidentiality of data being transmitted or stored, which is vital in various fields, including military and government applications, where securing sensitive information is paramount.

In contrast, the other options reflect characteristics that do not apply to the KYX-15. For instance, it being a handheld device does not align with its primary function, nor does it specifically process low-frequency signals, which relates more to radio technology rather than cryptography. Additionally, while enhancing antenna performance is an important aspect in telecommunications, it does not pertain to what the KYX-15 is designed to accomplish in the cryptographic domain. Thus, the right answer emphasizes the device's critical role in cryptographic operations.