What is prohibited when holding in icing conditions on the ERJ 170/175?

When holding in icing conditions on the ERJ 170/175, the use of flaps is prohibited. This limitation is crucial because deploying flaps in icing conditions can exacerbate ice accumulation on the wing surfaces, leading to a significant decrease in aerodynamic performance and increased risk of stall. Ice can disrupt the airflow over the airfoil, and with flaps deployed, the change in wing configuration can further enhance these risks. The aircraft's systems and operational guidelines are designed to minimize hazards related to flight in icing conditions, ensuring the safety and integrity of the aircraft during such operations.

The other choices, while they may have restrictions or specific guidelines associated with them, do not carry the same universal prohibition as flap usage in icing conditions does. Hence, the correct choice underlines the critical aspects of safely managing the aircraft's performance during icing encounters while holding.