The best solvent for the crystallization of organic solid is in which:

For crystallization, the ideal solvent is one where the compound is poorly soluble at room temperature but becomes highly soluble when heated. This allows the compound to dissolve upon warming and then crystallize upon cooling. Option B correctly describes this property. Options A and D are unsuitable because if the compound is highly soluble at all temperatures, it won't crystallize; if it is insoluble, it won't dissolve even when heated. Option C specifies a particular temperature (90°C), which is too specific and not generally applicable; the key is the behavior upon warming, not an exact temperature.