Which among the following database technologies have the most demand in the IT industry now-a-days?

If you're talking corporate IT, Oracle, SQL Server and DB2 are probably your best bets (in that order) unless you are intending to work as a webmaster in an intranet or company web site. In that case, MySQL is probably also a contender (tho likely one of the other three is also in use as the corporate back end which feeds the server database.)