Final answer:
The President and Vice President of the United States serve four-year terms, whereas the Secretary of State does not have a fixed term but traditionally serves for the duration of the presidential term.
Step-by-step explanation:
The tenure of service for the President and Vice President of the United States is a term of four years, according to Article II of the Constitution and has been established since the first president, George Washington. The Secretary of State, however, does not have a specified term length, and can serve under the pleasure of the President. Regarding presidential terms, the 22nd Amendment imposes a limit, stating that no person can be elected to the office of the President more than twice. Moreover, someone who has served more than two years of a term to which another was elected cannot be elected more than once.
The correct answer to the question regarding the tenure of service is: a. President: 4 years, Vice President: 4 years, Secretary of State: 4 years. It should be noted that while the Secretary of State's tenure is traditionally four years, coinciding with the presidential term, there is no fixed term limit, and they serve at the discretion of the President.