X dubbel ...

There is NO legal requirement to be charging

On parking places without chargers, these signs indicate only (hybrid)electric vehicles can park there

On parking places with a public charger, it means the car must also be connected to the charger.
It does NOT have to charge :!:

B. Yes, but only while charging.

This means that you can park your electric car in the designated space only when it is actively charging. At other times, parking there is not allowed. The information you provided aligns with this answer. If the test platform is giving "A. Yes" as the answer, there may be an error in the system. It's recommended to double-check with the official guidelines or consult the test provider for clarification.

[quote=Induja post_id=126920 time=1696836104 user_id=10348]
This supplementary traffic sign indicates that parking is only allowed for electric cars while the battery is charging. [/quote]

No, it doesn't
It limits parking to electric or electric/hybrid cars
It requires an electric or electric/hybrid car [u]to be connected[/u] to the public charging infrastructure, if present

Actually charging is NOT a requirement ...

Per the theory material under traffic signs :

This supplementary traffic sign indicates that parking is only allowed for electric cars while the battery is charging.

Bron: Gratis Rijbewijs Online - https://www.drivinglicence-belgium.be/theory/waiting-and-parking-part-3

Per the mock test question 12660 :
A parking space for electric cars with charging station.
Can you park your electric car there?

A. Yes.
B. Yes, but only while charging.

The answer comes up as A. Yes . This is confusing, what is the right answer?