Concept:The sentence needs the past participle form of "shake" to form a reduced passive adverbial clause.
Explanation:The complete idea is "Though he was shaken by opposition from his own supporters."
When the subordinate clause shares the same subject as the main clause, it can be reduced by keeping only the past participle.
"Shaking" is the present participle and expresses active, ongoing action, so it cannot show the passive meaning required here.
"Shook" is the simple past tense and cannot stand alone in a reduced clause without a subject and an auxiliary verb.
"Was shaken" cannot fill the gap because it is a complete finite verb phrase with no subject in this construction.
"Shaken" is the correct past participle of "shake."
The completed sentence is: "Though shaken by opposition from his own supporters, the Chairman pressed ahead with his plans."
Answer:C. shaken