**Core Concept**
The correct breathing circuit for a spontaneously breathing infant is one that ensures a safe and efficient delivery of oxygen while minimizing dead space and resistance. This is essential for maintaining adequate oxygenation and ventilation in infants, whose respiratory systems are still developing.

**Why the Correct Answer is Right**
The Mapleson A circuit, also known as the Magill circuit, is a type of breathing circuit commonly used for spontaneously breathing infants. It consists of a reservoir bag, a valve, and a tube that connects the bag to the patient's airway. The Mapleson A circuit is ideal for infants because it has a low dead space and resistance, which allows for efficient gas exchange and minimizes the risk of rebreathing carbon dioxide. The circuit also has a built-in valve that prevents the bag from collapsing, making it easier to ventilate the infant.

**Why Each Wrong Option is Incorrect**
**Option B:** The Mapleson D circuit is not suitable for spontaneously breathing infants because it has a higher dead space and resistance compared to the Mapleson A circuit.
**Option C:** The Bain circuit is typically used for controlled ventilation and is not the best choice for spontaneously breathing infants.
**Option D:** The Jackson-Rees modification of the Mapleson A circuit is not commonly used for spontaneously breathing infants and may have a higher dead space and resistance compared to the standard Mapleson A circuit.

**Clinical Pearl / High-Yield Fact**
When using a Mapleson A circuit for a spontaneously breathing infant, it's essential to ensure that the circuit is properly connected and that the bag is not over-inflated, as this can cause excessive pressure on the infant's airway.

**Correct Answer:** A. Mapleson A circuit.