a) ul
b) li
c) nav
d) both ul and li

Answer: d
Explanation: Conventionally, many web developers have use <ul></ul> and <li></li> tags to encapsulate navigation and then styled the elements appropriately as menu items. This seems to introduce quite a bit of ambiguity in markup because it may be difficult to determine the difference between a list that has links in it and a list that is simply navigation.