С – in. ‘To come in shape (of)’ is a common phrase that talks about the physical appearance of an object. ‘Comes with’ would mean being accompanied with something else, e.g. ‘This MP3 players comes with two AA batteries’.
C – artificial. Artificial means man-made. ‘Unnatural’ conveys the idea of contradicting the nature, which is not the idea of the sentence. ‘Fake’ has a strong negative connotation.’Crafted’ implies craftsmanship, e.g. made with care and high skill, usually refers to more complex objects.
D – believed. This is the most common impersonal structure to illustrate a commonly-held opinion among a group of people. ‘It is assumed’ means that no factual information to support this idea is present. Options A and C do not make any strong set phrases.
A – scale. The preposition ‘on’ helps us choose the right word. Generally, when talk about the volume of some operation (e.g. business or manufacturing) the word ‘scale’ should be used.
B – remains. Options C and D would require ‘as’ preposition to work. Option ‘A’ does not fit, as ‘leaves’ has a different meaning and requires an object.
D – measured. As we talk about size, we can’t count it. We can check it, but measuring is a more precise idea, while checking is more about making sure that something is right or wrong. ‘Estimating’ implies approximate calculation, which is not the approach here as we are talking about a very expensive commodity – diamonds.
B – considered. Options C and D need ‘as’ to work in this context. ‘Thought’ required ‘to be’ here.
A – into. ‘To break into’ in this context means splitting into smaller pieces or fragments.