Final answer:
Benztropine is the agent used intramuscularly to treat acute dystonias, as it is an anticholinergic that helps alleviate symptoms by blocking acetylcholine involved in muscle movement.
Step-by-step explanation:
The agent used intramuscularly to treat acute dystonias is benztropine. Acute dystonias are a form of muscle contraction disorder, which can manifest as muscle spasms, twitching, or abnormal posture. Benztropine is an anticholinergic agent that helps to relieve these symptoms by blocking the action of acetylcholine, which is involved in muscle movement. While benzodiazepines like diazepam are used for their sedative and anxiolytic effects, and drugs like haloperidol and propranolol have their own specific uses in different contexts, it is benztropine that is specifically indicated for the treatment of dystonias.