The senate positions were filled by allowing the U.S citizens to directly elect senators.
Option (c);
EXPLANATION:
Initially, before 1913, Americans don't elect the senators by voting. Instead, the State legislatures chose the respective senators. But, then the "Seventeenth" Amendment to the Constitution changed and they decided that citizens of America would directly elect Senators for the constitution.
The 17th Amendment to the Constitution states, a senator would be directly elected by the people and senators will each have a single vote to be cast in the election. When the government releases the vacancy for senate position the executive authority issues orders to fill these vacancies. Legislature of that state can appoint a temporary person to fill that position until the senator is chosen.