Hi Arghya,

I am not sure what you mean by apostrophe modifier.

Anyhow, choice A is incorrect because of pronoun reference error. Look at the pronoun "there" and think about what it refers to. It should refer to "Baltic Sea". Now ask yourself, can "there" refer to "Baltic Sea". No it cannot. This is because "Baltic Sea" is not used as a noun. It is used as an adjective here.

Furthermore, the reference of "which" modifier is ambiguous. Yes it can refer to "large concentrations". But at the same time, it can also refer to "Baltic Sea sediments", the closest noun. Thus, the modifier reference is not very clear. Because of this apparent ambiguity, this choice is incorrect.

Thanks,

Thanks @Frankenstein for clarifying this.

Well in my opinion, the modifier - findings consistent with...- modifies the preceding clause.
Such modifiers (Noun + Noun Modifiers) can either modify preceding clause or preceding noun. What it actually modifies depends on the context of the sentence. In this case this modifier modifies the preceding clause, implying that it provides some more information about scientists' observations. A way to verify this as shown:

Noun + noun modifier modifies the preceding clause if you can form two meaningful sentences in the following way.
1. Clause
2. This is (or these are) noun + noun modifier (where this refers to the clause)

Applying this framework to this sentence:
1: Scientists have observed large concentration of heavy-metal deposits in the upper twenty centimeters of Baltic sea sediments
2: These are findings consistent with the growth of industrial activity in the area.
The above two make complete sense.

Another example:
The sentence below is adapted from OG Verbal Review Purple Q#100 (with correct choice)
An asteroid bigger than Mount Everest slammed into North America, an event that caused the plant and animal extinctions that mark the end of the geologic era.

The noun + noun modifier in question are marked as shown.
1. An asteroid bigger than Mount Everest slammed into North America,
2. This was an event that caused the plant and animal extinctions that mark the end of the geologic era.
The above two sentences make complete sense.

For more details on this topic, you may view this link.