The absolute zero, i.e., temperature below which is not achievable, is about:

Absolute zero is defined as the lowest possible temperature where all molecular motion theoretically stops and a system has zero thermal energy. It represents the null point of the Kelvin scale, which is an absolute thermodynamic temperature scale. By international agreement, absolute zero is precisely 0 K on the Kelvin scale [1]. To convert between the Celsius and Kelvin scales, one must add or subtract 273.15; specifically, K = °C + 273.15. Therefore, 0 K is equivalent to -273.15 °C [1]. While it is physically impossible to reach absolute zero, scientists have achieved temperatures within millionths of a degree of this limit [2]. Among the given options, -273 °C is the closest approximation to this fundamental limit of coldness.

Sources

1. [1] https://www.oxfordsparks.ox.ac.uk/videos/towards-absolute-zero/
2. [2] https://www.nist.gov/blogs/taking-measure/how-low-can-temperature-go-lord-kelvin-and-science-absolute-zero