It is well known in the supermarket industry that how items are placed on shelves and the frequency of inventory turnovers can be crucial to profits.
(A) the frequency of inventory turnovers can be
the frequency of inventory turnovers should parallel the non underlined part expressing a similar idea (viz. How items are placed on shelves; so we should have the part connected with AND also beginning with HOW)
(B) the frequency of inventory turnovers is often
is often leads to SV error apart from parallelism issue as in choice A
(C) the frequency with which the inventory turns over is often
SV error;parallelism problem
(D) how frequently is the inventory turned over are often
OK, this one's close for sure - it solves parallelism issue as well as SV issue but introduces another problem - that of passivity(inventory turned over). On GMAT avoid passive constructions
(E) how frequently the inventory turns over can be
I knew (how...) is a subordinate clause. I narrowed it down to D & E.
D was tricky and i looked for subject verb inversion but there was a subject verb disagreement.
original: how frequently is the inventory turned over are often
new: how frequently the inventory is turned over are
As such...I chose E.