Constant exposure to our public figures’ foibles and flaws is at once humanizing and (i) __________; as we become sympathetic to their plights, we also become increasingly wary of their motives.

The sentence talks about the two consequences of the constant exposure to our public figures' foibles and flaws:

1. humanizing
2. _________

It then further elaborates (after the colon) the same idea by explaining in detail the two consequences

A. we become sympathetic to their plights
B. we become increasingly wary of their motives.

Now the ideas in 1 and 2 are parallel to the ideas in A and B respectively.

also,

idea 1 leads to idea A
idea 2 leads to idea B

The "humanizing" as a result of our exposure their flaws and foibles leads us to "becoming sympathetic to their plights"
Similarly, we need a word for the second blank which leads us to "becoming increasingly wary of their motives"

The word from the choice list is disillusioning.