What is rightmost sentential forms? I'm solving some past job interview problems.
I met an embarrassing question about compilers.

The question is :


  Consider the following grammar, with start symbol $E$:
  
  \begin{align*} E &amp;\rightarrow E \ast E\\  &amp;\quad \mid E ~/~ E\\ 
 &amp;\quad \mid E + E\\  &amp;\quad \mid E - E\\  &amp;\quad \mid (E)\\  &amp;\quad
 \mid a \mid b \mid c \mid \ldots \mid x \mid y \mid z \end{align*}
  
  The following strings are legal derivations from this grammar:
  
  
  $a \ast b + c$
  $( a - b ) \ast c$
  $a ~/~ ( b – c)$
  
  
  Which of the above are rightmost sentential forms?


When I was an undergraduate student, I learned that rightmost(leftmost) sentential form is result of rightmost(leftmost) derivation.
so, I think if $1,2,3$ are legal derivations form above grammar then absolutely all of $1,2,3$ are rightmost sentential form.
If I'm correct, above question is very worthless.

Is there any counter example that is not rightmost sentential form of the grammar $G$ although it is a legal derivation from $G$?

OR

Is there another definition about rightmost sentential form corresponding to above question? I have googled "rightmost sentential form" but there are few results and most of them is the definition that I already know.
I found other answers here confusing, so let me explain it clearly here-
Definitions:

A sentential form is any string derivable from the start symbol. Note that this includes the forms with non-terminals at intermediate steps as well.
A right-sentential form is a sentential form that occurs in a step of rightmost derivation (RMD).
A sentence is a sentential form consisting only of terminals

The examples in your question are all sentences. And since any way of derivation (including RMD) ultimately leads to a sentence, technically all sentences are right-sentential as well as left-sentential forms. So there isn't any counter example which you asked for.
Clarifying Examples : Page 2 of this link