Final answer:
An ionization chamber is the appropriate instrument to measure fluoroscopic scatter radiation exposure rate in an X-ray installation due to its accuracy in measuring ionization of air caused by radiation.
Step-by-step explanation:
To measure fluoroscopic scatter radiation exposure rate in an X-ray installation, the most suitable instrument would be an ionization chamber. While a Geiger counter can detect and measure ionizing radiation, it is typically less accurate for measuring rates of radiation exposure over a given area or volume. A scintillation detector, which involves a scintillator and a sensor, might also be used, but an ionization chamber, which directly measures the ionization of air due to radiation, is generally preferred for accurately determining exposure rates in medical settings.