Hi

experts please help clarify

Geologists once thought that the molten rock known as lava was an underground remnant of Earth's earliest days, sporadically erupting through volcanoes, but they now know that it is continuously created by the heat of the radioactivity deep inside the planet.

A. was an underground remnant of Earth's earliest days, sporadically erupting
B.had been an underground remnant of Earth's earliest days and sporadically erupted
C.was an underground remnant of Earth's earliest days, which sporadically erupted
D.would be an underground remnant of Earth's earliest days that sporadically erupted
E.was an underground remnant of Earth's earliest days, having sporadically erupted

In answer option C, what does which refer to? This is from MGMAT CAT

1) Is it a 100% rule that which "ONLY and ONLY" should refer to the immediately preceding noun. If yes then in option C which should refer to Earth's earliest days.

2)Or is it that "which" should refer to the "logical" preceding noun....in which case in option C which should refer to 'molten rock' and not to "Earths earliest days" because logically days cant erupt through volcanoes...

Which one of the above mentioned two options is correct?

3) For prepositional phrases, "Which" should refer to the entire prepositional phrase right? For eg ''A box of chocolate, which was kept on the table, ...."..Here what should 'which' refer to? Box, Chocolate or 'A box of chocolate'?

4) Subject verb agreement and use of which - For eg (A) "A galaxy of stars, which is the biggest in the universe,....." and (B) "A galaxy of stars, which are unusual in their size, ....."...Please throw some light both on the SV agreement concept and use of "which" concept for these sentences....Are both these sentences correct?

Experts Please give detailed explanation to clarify the use of "which" in different cases

Thanks a lot
Tarun