Correct

本 vs 本子

/ běn vs běnzi /
Strokes

Definition

本 and 本子 both Nouns, which means “book; notebook”.

Example

本 is an agglutinative word while 本子 is a free word.
本 cannot be used freely and usually is used with some other words, eg:
练习本 (an exercise-book)
日记本 (a diary-book)
笔记本 (a note-book)

while 本子 can be used independently.

In examples ① ② ③, 本 cannot be used, but 本儿 will do.
1
Zhège
这个
běnzi
本子
shì
zhàng
记账
yòng
de
  

The notebook is for keeping accounts.

2
mǎi
le
ge
běnzi
本子
  

He bought several notebooks.

3
Běnzi
本子
shàngmiàn
上面
拉拉杂杂
de
xiě
le
shǎo
不少
dōng
东西
  

There are many things written ramblingly on the notebook.