22.03.24: Admiralty Arch

The image is of Admiralty Arch in London. The inscription reads:

ANNO : DECIMO : EDWARDI : SEPTIMI : REGIS

VICTORIAE : REGINAE : CIVES GRATISSIMI : MDCCCCX :

Admiralty Arch was built in 1910.

M (1000) + D (500) + CCCC (400) + X (10)

It was built for Queen Victoria: VICTORIAE : REGINAE [dative case]

ANNO : DECIMO [ablative] : EDWARDI : SEPTIMI : REGIS [genitive]

• In which year ¦ of which monarch’s (rēgis) reign was it built?

The last part: CIVES GRATISSIMI

You've seen the word grātus before, meaning 'pleasing'. Here, it has another meaning of "grateful; thankful"; add -issimus and you have the way in which Latin expresses "most": CIVES GRATISSIMI i.e. (from) the most / very grateful citizens.