1 TORR is equal to
(A) $1mm{\text{ }}of{\text{ }}Hg$
(B) $10mm{\text{ }}of{\text{ }}Hg$
(C) $0.5mm{\text{ }}of{\text{ }}Hg$
(D) $5mm{\text{ }}of{\text{ }}Hg$

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Hint: Torr is a unit of pressure named after the scientist Evangelista Torricelli who discovered the principle of a barometer. Torr was named in the honor of the inventor of the barometer, Evangelista Torricelli. One unit of gas pressure is expressed as a millimeter of mercury (mmHg).

 Complete step by step answer:
Torr is a unit of pressure. It is defined as exactly \[1/760\;\] of a standard atmosphere. The relation between the torr and millimeter of mercury is the following:
\[{\text{1 Torr = 0}}{\text{.999999857533699 mm Hg}} \approx 1mm Hg\]
\[{\text{1 mm Hg = 1}}{\text{.000000142466321 Torr}}\]
1 torr is equal to the 1mm of Hg.
Hence, the correct option is (A) is correct.

Additional information:
Pressure (p) is the force applied perpendicular to the surface of an object per unit area.
Some other units of pressure are below.
The bar (denoted by the bar), defined as \[{\text{100 kPa}}\] exactly.
The atmosphere (denoted by atm), defined as \[101.325kPa\].
These pressure units are used in different places. Torr is widely used to measure atmospheric pressure, in engineering.
The S.I. unit of pressure is Pascal (denoted by Pa), which is equal to one Newton per square meter \[\left( {{\text{N/}}{{\text{m}}^{\text{2}}}} \right)\]. The pressure is a scalar quantity.

Earlier, one torr was intended to be the same as one "millimeter of mercury", but subsequent redefinitions of the two units made them slightly different.
It is one of the measuring units in a high vacuum.