Final answer:
Sodium is known to react with water in a highly exothermic reaction that can cause it to form a spherical shape due to the rapid release of hydrogen gas and heat.
Step-by-step explanation:
The student questioned which metal forms a sphere when reacting with water. Among the options provided, the alkali metals such as lithium, sodium, and potassium are known to react vigorously with water to produce hydrogen gas and a basic solution of the metal hydroxide. However, it is sodium that is well-known for its highly exothermic and sometimes explosive reaction with water, which can result in it forming a spherical shape due to the rapid release of hydrogen gas and heat as it skitters across the surface of the water.