The main difference between the sentences does not lie in the words you marked in bold, but in their locations.

In the 1st sentence, "Many of them ... ago," modifies the mountainous regions, meaning that the mountains were chiseled from solid rock. Since mountains occur naturally and are not usually chiseled from rock (unless chiseled is used figuratively), this modifier seems misplaced. Notice that a modifiers such as this always modifies what follows the comma.

In the 2nd sentence, ",many of which ... ago." is a relative clause that modifies the monasteries, indicating that the monasteries were the ones chiseled from solid rock. This modifier is correctly placed. A relative clause modifies only what comes immediately before it.

"Many of" is a quantifier (indicating quantity), applied to to the modifier (many of them) or the relative clause (many of which).

Hi Amit,
the original sentence is:

Many of them chiseled from solid rock centuries ago, the mountainous regions of northern Ethiopia are dotted with hundreds of monasteries.

(A) Many of them chiseled from solid rock centuries ago, the mountainous regions of northern Ethiopia are dotted with hundreds of monasteries.
(B) Chiseled from solid rock centuries ago, the mountainous regions of northern Ethiopia are dotted with many hundreds of monasteries.
(C) Hundreds of monasteries, many of them chiseled from solid rock centuries ago, are dotting the mountainous regions of northern Ethiopia.
(D) The mountainous regions of northern Ethiopia are dotted with hundreds of monasteries, many of which are chiseled from solid rock centuries ago.
(E) The mountainous regions of northern Ethiopia are dotted with hundreds of monasteries, many of them chiseled from solid rock centuries ago.


Can you clarify what is wrong with D compare to E? I know the both constructions that we need to use are:
Many of which are
Many of them

Is it because the tense ARE is wrong (should be WERE) ? or do you see another reason ?

Elad

Is the OA D?