**Core Concept**
Oxytocin is a hormone involved in various physiological processes, including uterine contraction during childbirth and milk letdown during lactation. Oxytocin antagonists are pharmacological agents that block the action of oxytocin, thereby inhibiting uterine contractions and milk letdown.

**Why the Correct Answer is Right**
The mechanism of action of oxytocin antagonists involves competitive inhibition of the oxytocin receptor, which is a G protein-coupled receptor. Oxytocin binds to this receptor, causing an increase in intracellular calcium and subsequent contraction of uterine smooth muscle. Oxytocin antagonists, such as atosiban, block this binding, preventing the increase in intracellular calcium and subsequent uterine contraction. This action is particularly useful in the management of preterm labor.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the action of oxytocin antagonists.
**Option B:** This option is incorrect as it does not accurately describe the mechanism of action of oxytocin antagonists.
**Option C:** This option is incorrect as it refers to a different class of drugs that affect uterine contractions.

**Clinical Pearl / High-Yield Fact**
Oxytocin antagonists are useful in the management of preterm labor, particularly in cases where tocolysis is required to delay delivery. However, their use should be carefully monitored due to the potential for rebound uterine contractions.

**Correct Answer: A. Atosiban.**