I will go with C on this.
C maintains parallelism in 2 ways. verbs and infinitives, hence is my choice.
Just because verbs are in parallel, does make a sentence correct. All other things need to be parallel too.
(A) by proposing imposing stiff economic sanctions on the country's export, and still others by demanding
-OUT, no verbs in 2nd and 3rd list, these lists are called sentence fragments.
(B) by proposing the imposition of stiff economic sanctions on the country's exports, and still others demand
-OUT, no verbs in 2nd, same as A
(C) propose imposing stiff economic sanctions on the country's exports, and still others are demanding
Keep this
(D) are proposing the stiff imposition of stiff economic sanctions on the country's exports, and still others are demanding
- OUT, the imposition of, weird construction
(E) propose the stiff imposition of stiff economic sanctions on the country's exports, and still others demand
- OUT, the stiff imposition of, weird construction again, we need "to impose".
Here is what I think, if you are using infinitives, they need to be parallel, you can definetly mix "to infinitive" with "ing infinitive" but care must be taken to seperate "ing infinitive" from gerunds.