**Core Concept:** Safe drugs for nursing mothers are those with minimal risk to the infant due to passive transplacental passage of the drug and minimal impact on milk production and composition.

**Why the Correct Answer is Right:** Paracetamol (Option D) is a common, safe analgesic and antipyretic drug, with minimal placental passage and minimal effects on milk production and composition. It has fewer adverse effects on the infant compared to other drugs, making it a suitable choice for nursing mothers.

**Why Each Wrong Option is Incorrect:**

A. Ibuprofen (Option B) is a nonsteroidal anti-inflammatory drug (NSAID) that crosses the placenta and increases the risk of bleeding in the infant due to its effect on platelet aggregation.

B. Codeine (Option C) is an opioid analgesic that crosses the placenta and can lead to respiratory depression in the infant. Opioids can also interfere with breast milk production due to their effect on prolactin levels.

C. Aspirin (Option A) is an NSAID that crosses the placenta and increases the risk of bleeding in the infant due to its effect on platelet aggregation. Aspirin also affects milk secretion and composition, making it an unsafe choice for nursing mothers.

**Clinical Pearl:** When prescribing drugs to nursing mothers, consider the drug's pharmacokinetic and pharmacodynamic properties, as well as the potential for side effects in the infant and interference with lactation. In general, paracetamol remains the safest choice for pain relief in nursing mothers.