Which of the following is a valid IPv6 address?

Option A contains the character 'T' in the group '54AT', and 'T' is not a valid hexadecimal digit, immediately invalidating this as an IPv6 address.

Option B contains 'K' and 'P' in the group '12KP', neither of which are valid hexadecimal digits, making this address syntactically impossible.

Option C (123.111.243.123) is a dotted-decimal IPv4 address using the wrong format and delimiter, and cannot represent an IPv6 address.

IPv6 addresses are composed of eight 16-bit groups expressed in hexadecimal, using only the digits 0-9 and letters A-F. Option D contains exclusively valid hexadecimal characters across all eight groups, making it the only syntactically correct candidate among the choices, even accounting for the non-standard delimiter formatting used in this question.