"since + present perfect...have begun"
This narrows the selection to C/D
"based on the performance of ~" is idiomatic.
My choice is (D)
can you give another example of the use of this idiom?
The pronoun "they" cannot be used for possessive noun "investment officers' fees".Only a possessive pronoun(their) can be used with a possessive noun.
Excellent catch. This was touched upon in ManhattanGMAT's SC book.