Final answer:
The programming concept most suitable for recursive directory search is Recursion, and the time complexity of binary search is O(log n).
Step-by-step explanation:
The concept most suitable for implementing a recursive directory search is c) Recursion. Recursive functions are designed to call themselves with a modified parameter until a base condition is met, making them ideal for navigating through nested structures like directory trees.
For the second part of the assignment, the time complexity of a binary search algorithm is b) O(log n). Binary search is an efficient algorithm that repeatedly divides the sorted list in half to find the target value, leading to a logarithmic time complexity.