Frustrated about this question about Power: Expert vs Formal

I am memorizing the types of power based on this diagram:


The easy way I remember is that formal and reward are the best for compliance, expert and reverent are the best for commitment.


In this question:

You are a project manager leading a cross-functional project team in a weak matrix environment. None of your project team members report to you functionally, and you do not have the ability to directly reward their performance. The project is difficult, involving tight date constraints and challenging quality standards. Which of the following types of project management power will likely be the MOST effective in this circumstance?

A. Referent
B. Expert
C. Penalty
D. Formal


The official answer is:


Reward and expert are the best sources of power. Reward is not listed as a choice.


Why???  I take the question to ask: which type of power will give you the most compliance: which, out of the options is D.  The official answer doesn't make sense "reward and expert are the best power" - according to what?  Reward gives better compliance, and expert gets better commitment - they have equal pros and cons with Formal and Referent respectively.  I am frustrated that some questions appear to be stubbornly by the book, and some answers look like someone just made up something on the spot because it sounds good.

I say this because I came across this question in a previous mock exam, so I remembered the motto "reward and expert are the best power", then I got a question wrong on a different exam because they were looking for "which power gives the most commitment".  So I remembered the table, and now I got this same question again in a 3rd mock exam (I forgot the wording the first time I was asked this question) and the table rules don't apply to this question.  It's so inconsistent!

There will be more cross-functional project. - Phillip Elden