Final answer:
Kim Philby is the British individual known by the initials "K.P." who spied for the Soviets within MI6, matching your description and born on January 1, 1912. He is the correct answer A) among the given options.
Step-by-step explanation:
The British individual who spied within MI6 for the Soviets and was born on January 1, 1912, is Kim Philby, designated with the option A) in your list. Kim Philby was a high-ranking member of British intelligence who notoriously worked as a double agent before defecting to the Soviet Union. He was a member of the covert Cambridge Spy Ring that operated in the UK during the early to mid-20th century. This ring included other spies like Donald Maclean and Guy Burgess, both noted in options C) and D) but they do not match the initials "K.P." Klaus Fuchs, mentioned in option B), was a German-born physicist who passed nuclear secrets to the Soviets but he was not the spy indicated by the initials "K.P." and his espionage activities were more associated with the Manhattan Project rather than direct work within MI6.