Is tightest upper bound equivalent to best case complexity

Question

In gate 2003 cse question, i read that tightest upper bound of selection sort is O(n), which occurs only when ( n-1) swaps.

 

My doubt is " tightest upper bound " is kuust another name of best case complexity?

Answer 1

tightest upper bound can be determind for 

a)worst case:eg quick sort O(n²)

b)avg case: eg.quick sort O(nlogn)

c)best case eg. quick sort O(nlogn)

In your case its for best case and depending upon condition its varies.