Which of the following is classified as a muscle relaxant?

Baclofen is classified as a muscle relaxant, specifically acting as a centrally acting muscle relaxant. It is primarily used to relieve muscle spasticity caused by conditions such as multiple sclerosis or spinal cord injury. Baclofen works by inhibiting the transmission of signals in the spinal cord, which helps reduce muscle tone and relieve spasms.

The other options listed do not serve the same purpose or function as muscle relaxants. Aspirin is a non-steroidal anti-inflammatory drug (NSAID) primarily used for pain relief and reducing inflammation. Amoxicillin is an antibiotic that is effective against bacterial infections but has no muscle relaxant properties. Hydrochlorothiazide is a diuretic that helps eliminate excess fluid from the body and is used to treat hypertension and edema, not muscle relaxation. Thus, Baclofen stands out as the substance that directly serves the role of a muscle relaxant.