Final answer:
The cabinet position that served as a springboard to the presidency in the early US was the Secretary of State, as evidenced by the fact that Thomas Jefferson, who held the position under George Washington, later became President.
Step-by-step explanation:
During the early years of the US presidency, the cabinet position that became a "springboard" to the presidency was D. Secretary of State. The Secretary of State was considered part of the "inner cabinet" and received significant attention from the president, Congress, and the media. George Washington's initial cabinet included Thomas Jefferson as Secretary of State, and indeed, Jefferson would later become President, exemplifying the role's potential as a launching pad for the presidency.
In contrast, for example, Albert Fall was Secretary of the Interior and was involved in a bribery scandal, which suggests that not all cabinet positions held the same level of prestige or potential as a springboard for higher office.