**Core Concept**
Detrusor instability, also known as overactive bladder, is a condition characterized by a sudden urge to urinate, often accompanied by incontinence. The detrusor muscle is the smooth muscle layer of the bladder wall responsible for bladder contraction. Management of detrusor instability involves pharmacological interventions that either relax the detrusor muscle or inhibit the neurotransmitters stimulating its contraction.

**Why the Correct Answer is Right**
The drugs used to treat detrusor instability typically belong to the classes of antimuscarinics (also known as anticholinergics) or beta-3 adrenergic agonists. These classes of drugs work by either blocking the action of acetylcholine on muscarinic receptors in the bladder, thereby reducing detrusor muscle contraction, or stimulating beta-3 adrenergic receptors, which also relax the detrusor muscle. Examples of antimuscarinic drugs used to treat detrusor instability include oxybutynin and tolterodine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because antispasmodic drugs like dantrolene primarily work by inhibiting calcium ion release from the sarcoplasmic reticulum, thereby reducing muscle contraction. However, they are not typically used to treat detrusor instability specifically and may have limited efficacy in this condition.

**Option B:** This option is incorrect because alpha-blockers like tamsulosin primarily work by relaxing the smooth muscle in the prostate and bladder neck, thereby reducing the pressure on the bladder and improving urine flow in men with benign prostatic hyperplasia. They are not typically used to treat detrusor instability.

**Option C:** This option is incorrect because beta-2 agonists like salbutamol primarily work by stimulating beta-2 adrenergic receptors, which cause bronchodilation and are used in the treatment of asthma and chronic obstructive pulmonary disease. They are not typically used to treat detrusor instability.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that antimuscarinic drugs can cause dry mouth, constipation, and blurred vision as side effects, which should be monitored in patients using these medications.

**Correct Answer: C. Salbutamol. Beta-2 agonist**