Ron Purewal says (and I post this here for the benefit of the group..I will take the KUDOS anyways ):
the correct answer to this problem is (e).

usually "thus" is used to mean roughly "therefore", BUT "thus" can mean "in the same way" or "in the same state".
for instance:
at maturity the caterpillar becomes a butterfly, and remains thus until its death. --> in this case, "thus" means "in the same state", and, therefore, means "as a butterfly".

same thing in choice (e), in which "thus" means "in the form of diamond".

this IS the intended meaning: AT THE TIME WHEN the pressure is removed, the substance stays in the form of diamond.

--

(d) is incorrect.

* "in this way" is unidiomatic here.
"in this way" can refer only to the MANNER IN WHICH AN EVENT OR ACTION OCCURS OR IS PERFORMED.
it CANNOT refer to the physical state of something. for instance, in the above example, you CANNOT use "in this way" to refer to the butterfly stage.

also:
* if you use "although", you are basically declaring that "the heat and pressure are removed".
in other words, "X, although Y" expresses the idea that X and Y are both facts (although they contrast with each other in some way).
this is not the intended meaning; "heat and pressure are removed" is a hypothetical that may or may not happen, and so should be introduced with "when" or "if" rather than "although".

-End of Ron's message

Thank you.