Ventilation in high rise buildings can best be accomplished by:

Ventilation in high-rise buildings is most effectively achieved through the strategic use of the building's HVAC (Heating, Ventilation, and Air Conditioning) system. This system is designed to circulate air throughout the building efficiently and can help control smoke and heat during an emergency. The HVAC system is built to manage air pressure and flow, which is crucial in a high-rise setting where the vertical nature of the structure can complicate traditional ventilation methods.

Using the HVAC system allows for precise control over ventilation rates and can help in maintaining tenable conditions for occupants while effectively clearing smoke and harmful gases. It can be adjusted to respond to the unique conditions of each floor, aiding in the protection of both occupants and first responders.

In contrast, opening all external doors and windows could create turbulent air flow and allow smoke to infiltrate other floors, potentially worsening the situation. Portable fans, while useful in certain scenarios, may not be effective in a high-rise building where airflow dynamics are complex and high winds could disrupt their function. Keeping elevators operational presents safety concerns, as they can serve as conduits for smoke if used incorrectly during a fire or emergency situation.

Thus, leveraging the building's HVAC system is the most reliable and safe method for ventilation in high-rise emergencies.