In the original sentence, Many of them... is modifying the hills, which should modify copper mines instead.
(A)
Many of them dug during the Greenwood copper boom a century ago, the hills to the east of Death Valley, Arizona, are dotted with hundreds of copper mines.
Incorrect because of modifier error.(B)
Dug during the Greenwood copper boom a century ago, the hills to the east of Death Valley, Arizona, are dotted with many hundreds of copper mines.
Here Dug... modifies hills, meaning hills were dug a century ago. Nonsensical. Eliminate.(C) Hundreds of copper mines, most of them dug during the Greenwood copper boom a century ago
, are dotting the hills to the east of Death Valley, Arizona.
No reason of comma after a century ago. Eliminate.(D) The hills to the east of Death Valley, Arizona, are dotted with hundreds of copper mines, most of them dug during the Greenwood copper boom a century ago.
Correct. Modifier placed correctly.(E) The hills to the east of Death Valley, Arizona, are dotted with hundreds of copper mines, most of which
are dug during the Greenwood copper boom a century ago.
are dug is a wrong form of tense when action was carried out in past. Eliminate.IMO D.
Bunuel
Many of them dug during the Greenwood copper boom a century ago, the hills to the east of Death Valley, Arizona, are dotted with hundreds of copper mines.(A) Many of them dug during the Greenwood copper boom a century ago, the hills to the east of Death Valley, Arizona, are dotted with hundreds of copper mines.
(B) Dug during the Greenwood copper boom a century ago, the hills to the east of Death Valley, Arizona, are dotted with many hundreds of copper mines.
(C) Hundreds of copper mines, most of them dug during the Greenwood copper boom a century ago, are dotting the hills to the east of Death Valley, Arizona.
(D) The hills to the east of Death Valley, Arizona, are dotted with hundreds of copper mines, most of them dug during the Greenwood copper boom a century ago.
(E) The hills to the east of Death Valley, Arizona, are dotted with hundreds of copper mines, most of which aredug during the Greenwood copper boom a century ago.