Which statement about time complexity is true?

• A. Time complexity measures memory usage.
• B. Space complexity describes runtime growth.
• C. Big-O notation is used only for memory usage.
• D. Time complexity describes how runtime grows with input size.

Answer: (D)

Time complexity describes how the runtime of an algorithm grows as the input size increases. This focuses on how many basic operations the algorithm performs as the amount of data grows, not on memory usage. Space complexity, on the other hand, is about memory usage, including any extra storage the algorithm requires beyond the input. Big-O notation captures these growth rates and is used to express both time and space, not only memory usage. So, describing how runtime grows with input size is the accurate statement.

For example, if an algorithm scans every element once, its time complexity is linear, O(n); doubling the input roughly doubles the running time.