Whose flag is this?

This three-blades motif is for the English county of Essex and characterises both its flag and its coat of arms.

Essex is a county to the North-east of London, one of the home counties. Essex borders Suffolk and Cambridgeshire to the north and Hertfordshire to the west. To the east lies the North Sea and to the south the estuary of the River Thames.

The county's coat of arms is of great antiquity. It comprises three Saxon seax knives (although they look rather more like scimitars), mainly white and pointing to the right arranged vertically one above another on a red background; the three-seax device is also used as the official logo of Essex County Council; this was granted in 1932. The emblem has been attributed to Anglo-Saxon Essex in Early Modern historiography.

Now for the other answer options:

-- The flag of the British army has two crossed swords, and is on a red background.

-- The flag of Saudi Arabia has just one sword, and on a green background.

-- The flag of Morocco has no blades but a star. It is on a red background.