The key word they give you in this sentence is nascent, which means
that it just came into existence. Therefore, it would make sense
that a definitive model would be premature. If you do not know what
nascent means, the second half of the sentence hints at that as well.
Only progress is essential, not a model. So, a tentative model would
not be decisive, rule out A. While a superfluous (unnecessary) model
would be inadvisable, that doesn’t have any context in the sentence; rule
out C. An impressive model is not vital judging by the second clause
of the sentence; rule out D. Building an irrelevant model doesn’t
really make sense in any context, so rule out E, leaving B.