**Core Concept**
The combination of L-Dopa and carbidopa in the treatment of parkinsonism is based on the pharmacological principle of aromatic amino acid decarboxylation. L-Dopa is converted to dopamine in the brain, but this conversion is also catalyzed by the enzyme aromatic L-amino acid decarboxylase (AADC) in peripheral tissues, leading to the formation of dopamine and other unwanted metabolites.

**Why the Correct Answer is Right**
Carbidopa, an AADC inhibitor, is given in combination with L-Dopa to prevent peripheral decarboxylation of L-Dopa, thereby increasing the amount of L-Dopa that crosses the blood-brain barrier and is converted to dopamine in the brain. This leads to a more effective and sustained improvement in parkinsonian symptoms. The combination of carbidopa and L-Dopa also reduces the peripheral side effects of L-Dopa, such as nausea and vomiting.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because carbidopa is not primarily used to increase the bioavailability of L-Dopa.
**Option B:** This option is incorrect because the synergistic effect of carbidopa and L-Dopa is due to the inhibition of AADC, not the enhancement of L-Dopa's pharmacokinetic properties.
**Option C:** This option is incorrect because carbidopa is not used to increase the half-life of L-Dopa.

**Clinical Pearl / High-Yield Fact**
To remember the purpose of carbidopa in L-Dopa therapy, recall that carbidopa inhibits the peripheral conversion of L-Dopa to dopamine, thereby "protecting" L-Dopa from premature decarboxylation and increasing its effectiveness in treating parkinsonism.

**Correct Answer: C.**