How to concatenate string having double quotes in C#

The problem is that that's an SQL command - so double quotes aren't applicable anyway, unless you want them as part of an actual string.
SQL uses single quotes for string delimiters, not double quotes, so to return a string literal all you would do is surround it with double quotes:

SELECT 'Hello world!';

If you are trying to "build a query" and return a column called Provider Review then you need to use either square brackets (SQL Server) or backquotes (Access):

SELECT [Provider Review] FROM MYTable

But the Query you show doesn;t make it at all obvious what exactly you want - it isn't valid SQL at all because Distinct is a keyword which needs to be followed by a column name rather than a comma.

If you need to put double quotes in a C# string literal, there are two ways
1) Escape it:

string s = "this is surrounded by \"double quotes\"";

2) Escape the string and double them:

string s = @"this is surrounded by ""double quotes""";

create table temp ("silly column name" varchar(30));
insert into temp ([silly column name]) values ('how horrible');
SELECT "silly column name" from temp;

You can user the query like to get in single line:

string sQury2 = " select Distinct";
sQury2 = sQury2 + " , name";
sQury2 = sQury2 + " , Decis.code \"Provider Review\"";

Result: sQury2 = select Distinct , name , Decis.code "Provider Review"
Use the following to break into lines like:

string sQury2 = " select Distinct" + "\r\n";
sQury2 = sQury2 + "   , name" + "\r\n";
sQury2 = sQury2 + " , Decis.code  \"Provider Review\"" + "\r\n";