A string x is the member of the language L, if all the turing machines M1, M2, ... Mn halt on that string.
Suppose, there is a string, let's take 0101, if all the TM halt on it, means they either accept or reject it then 0101 is a member of the language L.
Assume, there is another string, let's take 1010101 on which any TM doesn't halt, then neither we can accept 1010101 as a member of the language L nor we can reject it.
(A) is correct answer!