This all depends on what python version your are running:
If you install on a 64 bit Operating System you will get the option.
Since ArcGIS for Server 10.1 was released, the ArcPy library has been
accessible through a 64-bit platform. In addition, the 64-bit
Background Geoprocessing patch was released at 10.1 Service Pack 1,
which allows ArcGIS for Desktop users to utilize geoprocessing in
64-bit. This means that if you have ArcGIS for Desktop and either
ArcGIS for Server or the Background 64-bit Geoprocessing Patch
installed, you have two different installs of Python 2.7.
The default paths for those are:
C:\Python27\ArcGIS10.2 – (32-bit)
C:\Python27\ArcGISx6410.2 – (64-bit)
Background Geoprocessing (64-bit) is available as a separate installation on top of ArcGIS for Desktop. The following information only applies if you have the Background Geoprocessing (64-bit) product installed; otherwise, background processing is done in 32 bit.
What is 64-bit Background Geoprocessing?
The installation of the ArcGIS for Desktop—Background Geoprocessing
(64-bit) product replaces the regular 32-bit background processing
which comes standard with ArcGIS for Desktop. Using 64-bit processing
to perform analysis on systems with large amounts of RAM may help when
processing large data which may have otherwise failed in a 32-bit
environment. Since all execution is done in the native 64-bit space,
more system resources can be used.
Tools that execute in the background honor your current license. For
example, if you are licensed for ArcGIS for Desktop Standard, then
background execution will honor all tools that are licensed for
Standard. Background processing does not consume a second license;
only one license per machine is used.
If you want to return to 32-bit background processing, you need to uninstall the 64-bit background processing software.