Answer:
d. Higher savings rates.
Step-by-step explanation:
Rising levels of output per capital could be caused by higher saving rate. On the long-run, when the saving rate in an economy is high, there will be more money for capital investments which will result in an increased productivity, that is increased output per capital. Banks will have more money to loan out to firms for huge capital investment which will result in increased productivity.
On the contrary, lower saving rate will result in unavailability of funds for capital investments which can propel economic growth and thus reduce output per capital in the long-run.