**Core Concept**
The alpha subunit of G-protein plays a crucial role in signal transduction by facilitating the exchange of GDP for GTP and subsequently catalyzing the hydrolysis of GTP to GDP, thereby regulating the activity of downstream effectors.

**Why the Correct Answer is Right**
The correct answer, C. Breakdown of GTP to GDP, is the primary function of the alpha subunit of G-protein. Upon activation by a G-protein coupled receptor (GPCR), the alpha subunit exchanges its bound GDP for GTP, which causes a conformational change that activates the alpha subunit to catalyze the hydrolysis of GTP to GDP. This reaction is essential for the deactivation of the G-protein and the cessation of signal transduction. The alpha subunit's intrinsic GTPase activity is responsible for this process, which is critical for regulating the duration of the signal and preventing prolonged activation of downstream effectors.

**Why Each Wrong Option is Incorrect**
**Option A:** Binding of agonist is a function of the GPCR, not the alpha subunit of G-protein.
**Option B:** Conversion of GDP to GTP is the initial step in G-protein activation, but it is not a function of the alpha subunit.
**Option D:** Internalization of receptors is a process that occurs after receptor activation and is not directly related to the alpha subunit's function.

**Clinical Pearl / High-Yield Fact**
The intrinsic GTPase activity of the alpha subunit of G-protein serves as a "timer" for signal transduction, regulating the duration of the signal and preventing prolonged activation of downstream effectors.

**✓ Correct Answer: C. Breakdown of GTP to GDP**