Properties of auto-correlation functions

[anarachy]

Homework Statement

Determine which, if any, of the following functions have the properties of auto-correlation functions. Justify your determination.
[PLAIN]http://rndpakistan.com/projects/hswhs/images/ques.jpg

Homework Equations

[PLAIN]http://rndpakistan.com/projects/hswhs/images/autop.jpg

The Attempt at a Solution

 

[lewando]

Again, please share your thoughts so that help can be provided.