United Kingdom

A Hussar lieutenant

It is difficult to ascerain which regiment that young officer belongs to. It is however quite possible to tell a few regiments he does not belong to.
- The 10th Regiment's pouch belt had a very distinctive "chainy" pattern; so he's not 10th ;
- The 11th Regiment's collar was "V" shaped. So he's not 11th ;
- The 13th Regiment's collar colour was "buff" (actually white) So he's not 13th ;

I proceeded with order ; he's got a regular Light Dragoon pattern pouch belt :
- The 7th Regiment's pouch belt did not have chains and prickers. So he's not 7th, all the more as he doesn't wear his busby lines the same"short" way ;
- The 8th Regiment's pouch belt did not have chains and prickers. So he's not 8th ;
- The 14th Regiment's pouch belt was adorned with the Regimental Prussian Eagle (on the prickers' plate). So he's not 14th ; 
- The 15th Regiment's pouch belt did not have chains and prickers. So he's not 15th ; 
- The 18th Regiment's pouch belt did not have a central coloured stripe. So he's not 18th ;
- The 19th Regiment's pouch belt was decorated with a white central stripe. So he's not 19th ;

That leaves us with potentially the 3rd, 4th, 20th or 21st hussars (the latter being unlikely : the central stripe of their pouch belt was of a somewhat light blue). 

Before 1881 the badges of rank were worn on the collar. This photograph was certainly taken by the late 1890s or early 1900s.

Thanks to Charles Griffin for his help