The correct answer is B: Ayub Khan. Pakistan's Second Constitution was promulgated and enforced on June 8, 1962, by Field Marshal Ayub Khan. Ayub Khan had seized power in a military coup in October 1958, abrogating the first constitution of 1956 and imposing martial law. After four years, he introduced the 1962 Constitution, which established a presidential form of government and gave significant powers to the President, reflecting Ayub Khan's autocratic rule.
A: Iskander Mirza is incorrect. Iskander Mirza was the first President of Pakistan and was responsible for abrogating the 1956 Constitution and declaring martial law in 1958. However, he was quickly overthrown by Ayub Khan within weeks of declaring martial law, and therefore played no part in enforcing the 1962 Constitution.
C: Yahya Khan is incorrect. General Yahya Khan came to power much later, in 1969, after Ayub Khan resigned amidst widespread protests. Yahya Khan abrogated the 1962 Constitution, dissolved the assemblies, and imposed a new martial law. He was instrumental in the events leading to the 1971 war and the separation of East Pakistan, and later oversaw the first general elections in Pakistan's history.
D: Zulfikar Ali Bhutto is incorrect. Zulfikar Ali Bhutto became President in 1971 and later Prime Minister. He was a key architect of Pakistan's third constitution, the 1973 Constitution, which restored a parliamentary system. He was not involved in the enforcement of the 1962 Constitution, as he rose to prominence significantly after its promulgation.
Thus, the 1962 Constitution is intrinsically linked with Ayub Khan's era of presidential rule in Pakistan.