All are true about radiological features of the intestinal obstruction except:
The various threshold levels for bowel dilation are:
Small bowel -3cm
Cecum - 9cm
Distal large bowel -6cm (some books write 5cm)
Air-fluid levels in case of obstruction are seen on 'erect' abdominal films and not on 'supine', hence their absence on supine x rays would not rule out presence of obstruction.
About option 'c' 
Its very strange to combine 2 different pathologies in one generalization. Moreover volvulus can be of many types: gastric, midgut, cecal and sigmoid.
Whereas gastric, cecal and sigmoid volvulus can have specific plain x ray appearance, the midgut volvulus picture is usually non specific.
In mesenteric ischemia plain abdominal x-rays are relatively nonspecific and may be completely normal in more than it 25 percent of patients. Findings suggestive of acute mesenteric ischemia include the presence of an ileus with distended loops of bowel, bowel wall thickening (particularly prominent in acute mesenteric venous thrombosis), and/or pneumatosis intestinalis. Multidetector computed tomographic (CT) angiography and magnetic resonance (MR) angiography can be used to diagnose mesenteric ischemia in stable patients.
The small intestines are said to be dilated if the loops of bowel have a diameter of > 3cm.
The large bowel are said to be dilated if they are >5 cm for distal bowel and > 9 cm for proximal bowel.
Volvulus and mesenteric ischemia have characteristic signs on abdominal films.
The absence of air-fluid levels on plain supine radiographs does not rule out presence of an obstruction.