的确, 确实, 实在, and 真正: what's the difference?
A.的确   is a subjective and strong word to confirm. For your example in question, "没错，这本书____是我的", if you translate it into English, "Yes it is, actually this is my book ". Do you feel the mood is so strong? In another word, this book is mine, you never get a change to take it away, also "没错" is the key clue and makes this sentence much stronger. Eventually, the correct answer is "的确"！

B.实在 
Among native Chinese, few people speak "实在" as the meaning of "truly" or "actually". They more like to use it as adj, not adv. For an example, "这个人很实在" means "This guy is very good and not selfish".For adv, "实在"+ adj. is much easier for understanding，such as "实在漂亮". Note: 
"的确">"实在", if you don't know which word to use under different circumstance, try to use "的确" instead of "实在".

C.真正
There are only few specific phrase which you need to remember, "真正的男子汉","真正意义上的" . It is also not as often as used in daily life. 

As a result, all the key is to consider the situation and environment when you speak, there are always several key points and clue that would be found.

PS: If you want to say "really" in your spoken-Chinese, it is easy to use "真" + any word you want.  "真香","真漂亮","真性感","这本书真是我的" :)