a) rvalue (more specifically xvalue) since we return x as an rvalue-reference.

b) 

template <template <typename> typename T>
void fun(T<int> x)
{
  // ...
}

c) The diamond problem occurs when two superclasses have the same
base class thus giving rise to ambiguities in the subclass when using
members of the common base class.

d) It is a rule regarding name lookup. If a function cannot be
found by the normal lookup rules the compiler is allowed to look in
the namespaces of the objects passed in to the function as parameters.

e) That we wish the function or variable to be evaluated during
compile-time if possible.