What is the difference between isset() and other is_*() functions (is_alpha(), is_number(), etc.)?

A.
isset() is a function call and is_*() are not function calls

B.
is_*() are language constructs andisset() is not a language construct

C.
c)isset() is a language construct and is_*() are not language constructs

D.
d) is_*() return a value whereasisset() does not