I don't really like this question.
"They are not the same, but they both enforce uniqueness of the column on which they are defined"
assumes that they can both only be defined on a single column, whereas they can be multiple columns.
If this assumption is made then "The Unique key allows NULL values as a value" is indeed false, as you can have only one. But a unique constraint on 2 columns lets you have null in both values so the statement is true.
"by default a Primary key creates a Clustered Index"
is false. The default is only clustered if there is not already a clustered index.
create table test
(a int primary key,
b int unique clustered
will create a nonclustered primary key even though this is not specified.