In control flow terms, what signifies an event that executes only when a condition holds true?

The choice that signifies an event that executes only when a condition holds true is the "If - Then" statement. This construct is fundamental in programming and control flow logic, as it allows for conditional execution based on whether a specific condition is satisfied.

In an "If - Then" statement, the program evaluates the condition stated in the "If" clause. If this condition evaluates to true, the block of code under the "Then" clause is executed. This mechanism is essential for implementing decision-making processes within software, enabling the program to respond differently depending on varying inputs or states.

Since other options describe different programming concepts: a case statement is typically used for selecting among multiple conditions but does not exclusively execute based on a single condition; iteration refers to looping through a set of instructions repeatedly until a particular condition is met, rather than executing once based on a condition; and a process block generally refers to a collection of statements executed as a single unit, often without the conditional context. Thus, the "If - Then" structure is uniquely suited to handling conditional flow, making it the correct choice in this context.