In Apex, if a primitive data type variable like a Boolean is declared but not initialized, its value is null. The code initializes isOK without a value, so isOK is null. The if-else if block is evaluated sequentially. The first two conditions, isOK == false and isOK == true, both evaluate to false. The third condition, isOK == null, evaluates to true. Since theString == 'Hello' is also true, the entire third condition is met, and the code inside its block, x = 3;, is executed.

A. 1: This would be the result if the uninitialized Boolean isOK defaulted to false, which is incorrect in Apex; it defaults to null.

B. 2: This would be the result if isOK were explicitly initialized to true. It was not initialized, so its value is null.

D. 4: This would be the result if none of the preceding if or else if conditions were met, which is not the case here.

- Apex Developer Guide, "Data Types", Section: "Primitive Data Types". The documentation states, "If you declare a variable and don’t initialize it with a value, it will be null."

- Apex Developer Guide, "Expressions", Section: "Comparison Operators". The guide details how == works, including comparisons with null. The expression isOK == null will evaluate to true when isOK has not been assigned a value.