**Core Concept:** Test specificity is a measure of how well a test identifies people who do not have a disease. It is calculated as the proportion of people without the disease who test negative.

**Why the Correct Answer is Right:** We are given a screening test and the number of true negatives (TN), true positives (TP), false positives (FP), and false negatives (FN). The specificity (Sp) can be calculated using the formula: Sp = (TN / (TN + FP)). In this case, TN = 100 (120-20), TP = 15, and FP = 40 - TN - TP = 25. So, Sp = (100 / (100 + 25)) = 80%.

**Why Each Wrong Option is Incorrect:**

A. False: Specificity calculation involves true positives, true negatives, false positives, and false negatives, not just the number of true negatives.
B. False: The formula given in the explanation is correct for calculating specificity.
C. False: Specificity is calculated using the given formula, not just the number of true positives.
D. False: Specificity is calculated using the formula provided in the explanation.

**Clinical Pearl / High-Yield Fact:** In interpreting test results, specificity is important because it informs the probability that a positive test result indicates the absence of the disease. A high specificity indicates a reliable test.

**Correct Answer:** D. 80% (The specificity of the test is 80%)