Final answer:
The largest portion of the Supreme Court's caseload comes from the circuit courts and state supreme courts, with most cases being brought on appeal from lower courts. The Court selects cases based on a writ of certiorari.
Step-by-step explanation:
The largest portion of the Supreme Court's caseload comes from two primary pathways: the circuit courts or U.S. courts of appeals, and state supreme courts. Most cases heard by the Supreme Court are brought on appeal from lower courts, such as the U.S. Court of Appeals or state Supreme Courts. The Court selects cases based on a writ of certiorari, which is a request for judicial review and an order directing the lower court to send its records to the Supreme Court for review.