Final answer:
The rock is likely calcite that has metamorphosed into marble, since it has a sugary texture, is crystalline and white, and reacts with hydrochloric acid by fizzing.
Step-by-step explanation:
The rock described by the student is most likely calcite, which is a crystalline form of calcium carbonate (CaCO3). The descriptive keywords such as 'sugary, white surface', 'crystalline', and 'reacts with weak hydrochloric acid' match the typical characteristics of calcite that has undergone metamorphism. When metamorphosed, this rock can form marble, which has a sugary texture and reacts with HCl acid by fizzing due to the release of carbon dioxide.
Calcite is commonly found in various forms, including limestone and its metamorphosed counterpart, marble. Both of these can react with hydrochloric acid, which is a classic test for carbonate minerals.
It's also important to note that quartzite, another rock resulting from metamorphism, also has a sugary texture and can appear white. However, quartzite is composed of quartz and does not react with hydrochloric acid, which sets it apart from marble. The fact that the rock in question reacts with acid confirms that it is not quartzite.