Answer marked wrong BUT

Question

Es que
With es que we introduce a cause which is often at the same time a "justification or an excuse" for doing something or having done something.Direct from lesson
What is “As I don’t have any money”  if it not a justification?
What is “As I don’t have any money if it is not the cause”
IT POOR TEACHING IF YOU GIVE AMBIGUOUS QUESTIONS AND THE MARK THEM WRONG   
I have the same comments about the questions on Alguno

Answer

Hola Johan
If you read the whole lesson you will see how they are not always interchangeable. In this case, the only correct option is "como". We say in the lesson that:
1. Es que is an independent clause. It is introduced in the second sentence, not the first sentence. (not possible in this case)
2. Porque is more commonly/naturally used in the second part of the sentence, not the first part. (not possible in this case either)
3. Como is always introduced at the beginning and it is not possible to use it in the second sentence. (correct in this case)
We have also included a hint in the question saying "Bear in mind the most normal/natural order in Spanish".
I hope this clarifies it.
Un saludo
Inma

Answer

Are you therefore saying that es que is only used in answer to a question?

You seem to indicate that in the lesson examples the independent clause introduced by es que is the second sentence. You go on to say what distinguishes es que is that it is used as a second sentence. Does this mean that es que MUST always be introduced by a first sentence?

Answer

I mean that when you give an excuse/justification using "Es que",  "es que" is always found at the beginning of a sentence (an independent one). And this sentence explains the excuse to whatever is expressed in a first sentence.
With the other options (como and porque) you will normally find a sentence with a subordinate clause introduced by como or porque. See this example that we can express using the three structures:
1. -Como Alberto ha estado en Tenerife una semana, Alberto está muy moreno. 
(As Alberto has been in Tenerife for a week, Alberto is very tanned)
Subordinate clause "como Alberto ha estado en Tenerife una semana..."
2. -Alberto está muy moreno porque ha estado una semana en Tenerife.
(Alberto is very tanned because he's been in Tenerife for a week)
Subordinate clause "porque ha estado en Tenerife una semana"
3.  -Alberto, estás muy moreno. -Es que he estado en Tenerife una semana.
-Alberto, you are very tanned. -The thing is I've been in Tenerife for a week.
Too independent clauses. No subordinate clause.
The "es que" sentence comes after a previous one because as it is expressing a cause/justification, a previous comment is needed to give that justification. 
Inma