According to Interstudy, a nonprofit organization that studies health

tonymouse , I am not sure which part of the clause that you quote does not make sense.

Maybe the words that are missing after 73 percent and will be?
Words have been omitted. This construction is called ellipsis or elliptical construction.
We can drop words in English.
Elliptical construction is among the hardest subjects to tackle, even for native speakers.

My shortened version of correct answer (C):
According to Interstudy, only 36 percent of HMO’s were profitable last year; it estimates that this year 73 percent will be.

When I write in the words that have been omitted, the sentence is:

According to Interstudy, only 36 percent of HMOs were profitable last year;
it[Insterstudy] estimates that this year 73 percent [of HMOs] will be [profitable].

• it = Interstudy (the non-profit organization that does this kind of analysis)
A pronoun such as IT can refer to a noun in a previous clause even if the two clauses are separated by a semicolon.

• estimates that this year 73 percent [of HMOs]
The "of HMOs" is omitted after the second percentage because we know that we are comparing
profitability of HMOs. (What else would 73 percent refer to if not HMOs?)

We can omit words as long as meaning is clear and we do not break
any grammar rules such as parallelism.

• will be [profitable]
The adjective and subject complement "profitable" is dropped the second time
that HMOs are . . . well, they are not mentioned explicitly, but HMOs are referred to by the words "73 percent."

We omit words because repetition can create leaden prose, or wordy prose, or both.

I hope that I cleared up whatever was confusing about the clause that you highlighted.

If not, ask a question that is a little more specific, and tag me or someone else.