My answer is D....
Firstly, the noun modifier initially modifies the noun stones not the scientists so stones should follow it, thus...
A and C are eliminated
E sounds awkward remove it...
now if we look at B present continuous followed by simple past never heard of it plus "puzzling to scientists" that also sounds awkward while as
In D
if main clause is simple present then the dependent clause can be simple past if "it shows a past time action" that is a rule
here,
...
Firstly, the noun modifier initially modifies the noun stones not the scientists so stones should follow it, thus...
A and C are eliminated
E sounds awkward remove it...
now if we look at B present continuous followed by simple past never heard of it plus "puzzling to scientists" that also sounds awkward while as
In D
if main clause is simple present then the dependent clause can be simple past if "it shows a past time action" that is a rule
here,
...
