Replace the single-quotes with square brackets like so:
string tblname = students;
string query = "select * from [" + tblname + "] where rollno=123";
By putting single-quotes, you are basically telling your database engine to select everything from a string. Probably not what you want. MSAccess interprets anything inside square brackets as a database object (table, column, etc.). You could simply leave off the brackets altogether, but then if your variable tblname contains any spaces or special characters (which Access allows) then you'll get an error.