Refer to figures.
Carburettor icing occurs due to the shape of the carburettor (venturi), which increases speed and decreases static pressure, thereby decreasing temperature also. A further effect is the vaporisation of the fuel in that section, which takes latent heat out of the air in the carburettor, further decreasing the air temperature.

This can lead to ice building up in the air lines, mainly anywhere the airflow direction is changed, such as bends and restrictions. This can happen at outside air temperatures even up to +30°C, given the right humidity and engine throttle settings (worst at low power settings).

Regarding this question, Carburettor icing is most likely to occur at low power settings, such as during descent, when the throttle is partially closed. In this situation, the pressure drop in the carburettor venturi is greater, leading to more significant cooling of the air-fuel mixture. If the surrounding air is also moist and within the critical temperature range (typically between -5°C and +15°C), conditions become ideal for ice formation.