**Core Concept**
The question is testing the ability to diagnose pulmonary embolism (PE) in a patient with a high risk of thromboembolic events, such as cancer patients undergoing treatment. The underlying principle is that a specific investigation can be used to rule out PE in this context.

**Why the Correct Answer is Right**
The D-dimer test is a useful investigation in ruling out PE in patients with a low pre-test probability of disease, such as those with a normal chest X-ray and no clinical signs of DVT. A negative D-dimer test has a high negative predictive value (NPV), meaning that it can safely rule out PE in these patients. The D-dimer test measures the level of fibrin degradation products in the blood, which are elevated in the presence of a thrombus.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not a specific investigation for ruling out PE, as it may be normal even in the presence of a significant embolus. While a chest X-ray can show indirect signs of PE, such as the Westermark sign or Hampton's hump, it is not a reliable test for ruling out PE.

**Option B:** Echocardiogram may show signs of right ventricular strain or dysfunction in the presence of a large PE, but it is not a sensitive test for ruling out PE, especially in patients with smaller emboli.

**Option C:** CT pulmonary angiogram (CTPA) is a sensitive and specific test for diagnosing PE, but it is not the best choice for ruling out PE in a patient with a low pre-test probability of disease, as it involves radiation exposure and is more expensive than the D-dimer test.

**Clinical Pearl / High-Yield Fact**
In cancer patients undergoing treatment, the risk of thromboembolic events is increased due to the release of pro-coagulant factors from the tumor. A one-time D-dimer test can be used to rule out PE in these patients, but repeat testing is not recommended due to the high risk of false positives.

**Correct Answer:**
D-dimer test.