Pressure of the order of 10''' torr can be measured by

Answer: Option D

Pressures of the order of 10⁻⁷ torr (very high vacuum) are extremely low and cannot be measured by ordinary pressure-measuring devices.

A Bourdon tube is used for measuring relatively high pressures and is unsuitable for vacuum measurements.

A micro-manometer is used for small pressure differences but not for very high vacuum ranges.

A Pirani gauge can measure low pressures only up to about 10⁻³ torr.

An ionisation gauge operates on the principle of ionizing gas molecules and is specifically designed to measure very low pressures (high vacuum) of the order of 10⁻⁷ torr and below.

Hence, the correct answer is ionisation gauge.