Hint: The stimulus of cold treatment which is known as vernalization, is readily perceived by shoot tips, embryo tips and other meristematic regions of the plant.
Complete answer: The process of vernalization can be understood as the conversion of the digital epigenetic data stored at the cellular level into a quantitative response of the plant.
Plants generally sense conditions of the environment through the organs such as leaves, but the flowering development takes place in the stem cell niche at the apex. In simpler words, vernalization is basically the quantitative or the qualitative dependence of plants on being exposed to a fairly low temperature, for the plant to flower or in other words we can say it is the indirect stimulating effect of low temperature on subsequent flower formation in high temperatures by exposing the plant to these artificial conditions.
Temperature largely affects flowering, metabolic activities, and even germination of seeds in plants. The site that perceives the external cold stimulus can vary from one plant to another: it could either be the apical meristem present in the shoots, or even in some cases the germinating seed.
So, the correct answer is ‘Shoot apices’.
Note: In order to devernalize a plant it is usually exposed to relatively low and high temperatures subsequent to vernalization. Typically, vernalization temperatures have a narrow range i.e. between 1 and 7 degrees Celsius.