In case elective surgery of Ca sigmoid colon, which of the following should be done -
The principal treatment of colon cancer is resection of the lesion along with its lymphovascular supply.
About preoperative bowel preparation Sabiston 18/e p266 writes-

"Mechanical bowel preparation with the addition of oral antibiotics was the standard of care for several decades for any intestinal surgery. More recent studies have evaluated the need for both oral antibiotics and mechanical cleansing. Oral antibiotics confer no benefit to the patient and may increase the risk for postoperative infection with Clostridium difficile. In addition, although it seems intuitive that removal of bulk fecal material would decrease the risk for anastomotic and infectious complications, the opposite is true. A recent meta-analysis showed that both of these events are not decreased and may be increased with mechanical cleansing."

Prophylactic antibiotic
a broad-spectrum parenteral antibiotics is administered just prior to the skin incision.
there is no proven benefit of using antibiotics postoperatively after an uncomplicated colectomy so must not be continued beyond the day of surgery.
Mechanical bowel wash
Broad spectrum antibiotic given 48 hours before operation
Broad spectrum antibiotic at the time of operation
All of the above