What is a sticky end?

a. The exposed unique end of DNA cut by a restriction endonuclease (enzyme)

b. The restriction site of a plasmid that is recognized by a restriction endonuclease (enzyme)

c. The 3' or 5' end of bacterial DNA that is cut by DNA ligase

d. The top or bottom edge of an agarose gel, associated with gel electrophoresis, that tends to be sticky