The clock in a frequency counter normally uses a:

A frequency counter requires a highly stable and precise internal time base to accurately measure incoming signals. To achieve this necessary precision, the clock circuit uses a quartz crystal oscillator. Quartz crystals have excellent frequency stability over time and resist drifting caused by temperature variations far better than standard LC (inductor-capacitor) or RC (resistor-capacitor) circuits. Distractors like a free-running multivibrator or a Hartley oscillator are incorrect because they are significantly less stable and prone to thermal drift, making them entirely unsuitable as a measurement reference.