Question

What will best help markus understand the central idea as he reads

Answer 1

Normally what helps someone understand a central idea of something is the reoccurring subject. If the author repeats the same thing of a similar theme then it is likely that the reoccurring subject is the theme. 

For example in Stephen king's "IT" the theme continuously revolves around missing kids and a mysterious clown stalking the children, therefore, the central idea is the clown and missing children.

Answer 2

Finding a single concept that links the lines togather