On frequency
Select the correct readback.
Why exact readbacks matter
What must always be read back
Per ICAO Doc 4444 (PANS-ATM) and EASA's SERA.8015 "Content of read-back", certain items must be read back verbatim: the QNH, taxi routing to the holding point, runway crossing clearances, and take-off and landing clearances. A generic "Roger" or "Wilco" does not satisfy these, the controller needs to hear the specific numbers back to catch an error before it becomes a runway incursion.
"Wilco" vs a full readback
"Wilco" ("will comply") is fine for instructions that don't carry one of the mandatory items above, for example acknowledging a general heading or altitude change in some contexts. It is not a substitute for reading back a runway number, a holding point, or a clearance to cross or use a runway.
Why runway crossings get special attention
A significant share of runway incursions trace back to a taxi or crossing clearance that was assumed rather than confirmed. Reading back "crossing runway [X]" explicitly gives the controller one last chance to catch a misunderstanding before an aircraft ends up somewhere it shouldn't be.
A generic trainer, not your local procedures
"Sternfeld" and the callsign here are fictional, and phraseology can vary slightly between EASA member states and individual units. This tool teaches the general ICAO/EASA readback discipline, not a specific airfield's procedures, always follow your instructor, your examiner, and the controller actually on frequency.