What is standard temperature?
A ${\text{ - 273 K}}$
B ${\text{100 K}}$
C ${\text{273 K}}$
D ${\text{373K}}$

Hint: Standard temperature pressure is the set of standard conditions for experimental measurements that allow comparisons to be made between different data sets.

Complete step by step answer:
Every experiment conducted provides us a data. Any relevant calculation may depend on certain environmental factors such as temperature, pressure and sometimes humidity. It is possible to do experiments at very low pressure or zero pressure if in a vacuum or at very high pressure, experiments can be done with very low temperature or at very high temperature using a flame or in explosive condition but most of the experiments are done in regular atmospheric conditions which can be referred as standard conditions. These are standard sets of conditions for experimental measurements by the International Union of Pure and Applied Chemistry that allow easy comparison to be made between data sets.
The first set is STP or standard Temperature Pressure this refers a standard temperature of ${\text{0}}{{\text{ }}^{\text{o}}}{\text{C}}$ or ${\text{273 K}}$ and a pressure of ${\text{1 atm}}$
Standard temperature and pressure, agreed upon temperature and pressure for describing chemical and physical processes
The standard temperature is ${\text{273 K}}$

So, the correct answer is Option C.

Additional Information:
Sometimes when people refer to standard conditions they mean standard state condition
This includes a temperature of ${\text{273 K}}$ and a pressure equal to atmospheric pressure i.e., ${\text{1 atm}}$ or ${\text{101}}{\text{.3 kPa}}$. These sets of conditions can simplify the math needed to do important calculations.
Conversion of $^{\text{o}}{\text{ C}}$ to ${\text{K}}$ can be done by simply adding 273.15 to the $^{\text{o}}{\text{ C}}$ value
${\text{K = }}{{\text{ }}^{\text{o}}}{\text{C + 273}}{\text{.15}}$
Conversion of ${\text{K}}$ to $^{\text{o}}{\text{C}}$ can be made by simply subtracting 273.15 from the ${\text{K}}$ value
$^{\text{o}}{\text{C = K - 273}}{\text{.15}}$

Note: Standard temperature pressure is used in much tabulation and in thermodynamic calculations. The properties of density, boiling point, viscosity show a variation with temperature and pressure. Thus the calculations becomes easier while having a standard set of conditions