IMO D is the correct answer. Explanation:
(A)
overwhelmed hospital wards and snaking jobless lines have flickered across the world, people on the European side of the Atlantic are - Overwhelmed is a feeling. It is also an adjective, but here overwhelmed is not conveying the meaning which it supposed to do.
(B) overwhelmed hospital wards and snaking jobless lines
has flickered across the world, people on the European side of the Atlantic were - Images can not be followed by singular 'has'.
(C) overwhelming hospital wards and snaking jobless lines have flickered across the world, many people on the European side of the Atlantic
is - Many people can not be followed by singular 'is'.
(D) overwhelming hospital wards and snaking jobless lines have flickered across the world, people on the European side of the Atlantic are - Perfect choice
(E)
hospital wards that are overwhelming and snaking jobless lines have flickered across the world, many people on the European side of the Atlantic
is - Lengthy and error same as option 'C'