Final answer:
An erection requires sufficient blood hydrostatic pressure, the presence of nitric oxide, and an intact sacral spinal cord to initiate and maintain the physiological changes needed for this process.
Step-by-step explanation:
For erection to occur, D) all of the above elements are necessary:
A) there must be sufficient blood hydrostatic pressure to engorge the erectile tissues; B) nitric oxide (NO) must be present to induce the relaxation of the smooth muscles that surround the penile arteries, leading to vasodilation; and C) the sacral spinal cord must be intact to properly relay the nervous signals essential for initiating and maintaining an erection.