what is the time complexity

Question

Answer 1

See here

A) is wrong. n² + n can not have lower bound n.

B) well this very much wrong. For being Theta(n) , we need n to be lower bound. That is not possile

C) This is correct. n² + n has lower bound n² .
 D) This is wrong.n² + n has upper bound n²

Upshot, Dont believe answers in Test series ! .

Answer 2

f(n) is greater than n. 

g(n) is grater than n² 

f(n) +g(n)= max (f(n),g(n)) = lower bound n²