No option for 64-bit OS VirtualBox guest OS in Ubuntu 14.10 Dell OptiPlex 755

Solution 1:

Enable Intel (R) Virtualization Technology and Intel (R) VT-d Feature in BIOS settings.

Solution 2:

In the BIOS I had to change the following settings

Security > Execute Disable (set to On)
Performance > Virtualization (set to On)
Performance > VT for Direct I/O Access (set to On)
Performance > Trusted Execution (set to Off)

In my case first 3 i have already turned on, and what i had to turn off was Performance > Trusted Execution

Thanks to the article that pinpoited exact issue with bios on dell optiplex755 http://lifeofageekadmin.com/enable-virtualization-on-dell-optiplex-755-to-run-oracle-virtualbox-4/

Now everything is ok