Choice "B" is correct. The U.S. Treasury bill is considered to be a cash equivalent item so purchasing the T-bill merely changes the form of cash held, it does not change the cash position of the entity. Thus, the purchase is not reported on the statement of cash flows.
Choice "d" is incorrect. Changes between cash and cash equivalent items do not affect the cash position of the entity and are not reported as an outflow from operating activities on the statement of cash flows.
Choice "a" is incorrect. Changes between cash and cash equivalent items do not affect the cash position of the entity and are not reported as an outflow from investing activities on the statement of cash flows. However, purchase of other forms of debt instruments is an investing activity.
Choice "c" is incorrect. Changes between cash and cash equivalent items do not affect the cash position of the entity and are not reported as an outflow from financing activities on the statement of cash flows.