Name the metal which floats on the water without any apparent reaction with water.
a.) Magnesium
b.) Mercury
c.) Gallium
d.) Lithium

Hint: This element is from the 3rd group of the periodic table. Any compound that has density less than water will float. Now you can easily find your answer.

Complete step by step answer:
The concept is pretty simple: anything that is heavier than water will sink down and anything which is lighter than water will float on it.
There are six alkali metals - part of the Group 1 metals: Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These metals float on water because of two reasons: they are very lightweight metals and they react very aggressively with water to form hydrogen gas which helps them to float in water.
Na and K (sodium and potassium) react rapidly with water but float on water. Lithium (Li) reacts slowly with water (exothermic).
Lithium, sodium, and potassium have low densities and float on water. Rubidium and Caesium are denser and sink in water.
Lithium has a density of 0.53 g/cc it will float on water and any other metal with a density even slightly greater than 1 g/cc will sink.

Therefore, the correct answer to this question is option D.

Note: We should know that Gallium (density 5.91 g/cc) floats on its liquid as it contracts as it melts and expands when it freezes. Further, it is less dense than liquid gallium (density 6.1 g/cc). It is a property shared only by a couple of other elements, Bismuth and Antimony.
Gallium does not react with water at temperatures up to 100 $^{ 0 }C$.