Encryption that uses 16-character keys is known as strong 128-bit encryption strong 256-bit encryption military-strength 512-bit encryption military-strength 1,024-bit encryption

Strong 128-bit encryption.

Explanation: An individual character corresponds to 8 bits . So if there are 16 characters then,

168=128 bits keys

This makes the 128 bit key encryption the strongest amongst all other options.It is strongest because it is almost impossible to decode the 128-character key  by any method e.g.-brute-force method.

Other options are incorrect because the characters given by the other bits are as follows:  =32 characters

                         =64 characters

                         =128 characters  

Thus, the strongest character key is of 128 bits. for cipher.