"is an active process" is preferred over "as an active process". C and D
remain. C has an awkward use of "that".

Is it D ?

I go with Jay....It should be D....

OA???

IMO D

A) "Indicates" is singular when the subject "findings" is plural
B) not parallel
C) not parallel. There is also a pronoun reference error with "it"
E) Indicative of X as Y is idiomatically incorrect

Although they all seem to have some flaw in their construction I'd go with D b/c it seems least likely to be incorrect.

Indicating + Clause is correct

In April 1997, Hillary Rodham Clinton hosted an all-day White House scientific
conference on new findings that indicates a child's acquiring language, thinking, and
emotional skills as an active process that may be largely completed before age three.
Option A.
the verb 'indicates' does not follow parallelism with 'new findings' am -ing ending verb would have been a perfect answer.Thus, this option is incorrect.

Option B. the adjective 'indicative' here does not follow parallelism and cannot be used in syn with 'new findings'

Option c.
'to indicate' well this looks cool to some extent but a proper gerund would perfectly fit into space. This option is incorrect.

Optipon D.
Exactly! we are in the right place. the use of 'indicating' here clearly express the parallelism between 'new findings and indicating'THIS IS THE CORRECT ANSWER.

oPtion E.
This answer suffers the same fate as option B. So, it is practically wrong to use 'indicative'