Out of the following oscillators, one is NOT, by itself, considered a high-stability reference:

A Voltage-Controlled Crystal Oscillator (VCXO) is designed so its operating frequency can be pulled or shifted over a small range by applying an external control voltage. Because its output relies on this variable external input, it is intentionally adjustable and not inherently stable enough to be an absolute reference by itself. In contrast, OCXOs (Oven-Controlled) and TCXOs (Temperature-Compensated) use strict environmental controls to maintain extreme frequency stability despite ambient temperature changes. GPSDOs use absolute timing signals from GPS satellites to continuously discipline the oscillator. Therefore, the VCXO stands out as the only oscillator in the list that is not, entirely on its own, treated as a high-stability reference standard.