Answer:
The 3 correct answers are 1, 3, and 5:
(Correct) A cross section parallel to the base is a rectangle measuring 15 inches by 8 inches.
(Wrong) A cross section parallel to the base is a rectangle measuring 15 inches by 6 inches.
(Correct) A cross section perpendicular to the base through the midpoints of the 8-inch sides is a rectangle measuring 6 inches by 15 inches.
(Wrong) A cross section perpendicular to the base through the midpoints of the 8-inch sides is a rectangle measuring 4 inches by 15 inches.
(Correct) A cross section not parallel to the base that passes through opposite 6-inch edges is a rectangle measuring 6 inches by greater than 15 inches.