Often major economic shifts are so gradual as to be indistinguishable at first from ordinary fluctuations in the financial markets.
Option Elimination -
(A) so gradual as to be indistinguishable - so (gradual - cause) as to (be indistinguishable - effect). ok
(B) so gradual they can be indistinguishable - "that" is missing to complete the "so..that" idiom. Also, a can is not required.
(C) so gradual that they are unable to be distinguished - means as if shifts are somehow unable to do something - wrong meaning.
(D) gradual enough not to be distinguishable - "enough" shows sufficiency. E.g., Tom studied diligently enough to earn a high GMAT score and secure admission to a top-tier business school. Here, we aren't trying to say that the shifts are above some threshold not to be distinguishable. We are just saying that the shifts are so gradual (no threshold) as to be indistinguishable.
(E) gradual enough so that one cannot distinguish them - same as D. Moreover, why add "one" and "them" unnecessarily?