Decision coverage requires that every decision point in the code is tested for both its TRUE and FALSE outcomes. The provided code has two decision points:

1. if orderquantity >= 20

2. if orderquantity >= 100

To achieve 100% decision coverage, we need test cases that cover all possible outcomes:

Test 1: orderquantity < 20 (e.g., 15). This covers the FALSE outcome of the first decision. The second decision is not reached.

Test 2: orderquantity >= 20 AND orderquantity < 100 (e.g., 50). This covers the TRUE outcome of the first decision and the FALSE outcome of the second (nested) decision.

Test 3: orderquantity >= 100 (e.g., 110). This covers the TRUE outcome of the first decision and the TRUE outcome of the second decision.

A minimum of three tests are therefore required.

B. 4: This is more than the minimum number of tests required. Decision coverage seeks the most efficient set of tests to cover all decision outcomes.

C. 5: This is incorrect and likely confuses decision coverage with a more exhaustive technique like path coverage, which would not even require this many tests here.

D. 2: Two tests are insufficient. For example, testing with values 15 and 110 would cover the FALSE outcome of decision 1 and the TRUE outcomes of both decisions, but would miss the FALSE outcome of the second decision.

1. ISTQB® Certified Tester Foundation Level Syllabus (2018, v3.1.1): Section 4.2.2, "Decision Coverage," defines the technique as exercising all decision outcomes (both true and false). The logic applied follows this principle to determine the minimum number of tests for the nested structure.

2. Jorgensen, P. C. (2013). Software Testing: A Craftsman's Approach, Fourth Edition. CRC Press. Chapter 8, "Graph-Based Testing," explains how to model code with a control flow graph. For the given code, the graph would show three distinct paths are necessary to cover all edges representing the decision outcomes, confirming the need for three tests. (DOI: https://doi.org/10.1201/b15696)

3. Graham, D., Van Veenendaal, E., Evans, I., & Black, R. (2012). Foundations of Software Testing ISTQB Certification, 3rd Edition. Cengage Learning. Chapter 4, Section 4.3, "White-box or structure-based techniques," details decision coverage. The examples illustrate that for nested decisions, test cases must be chosen to specifically trigger each outcome, including those within the nested logic.