Why is strontium more reactive than calcium but less reactive than rubidium?

Sr is  below Ca and to the right of Rb in the Periodic Table (see the attached image).

Reactivity increases from top to bottom of a Group. Sr is below Ca, so it is more reactive than Ca.

Reactivity decreases from left to right in a Period. Sr is to the right of Rb, so it is less reactive than Rb.

i think   the answer is c hope i'm right

answer is: naturally occurring solid, inorganic compound or element is mineral.

minerals usually have crystalline form, that means than crystal constituents (atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

for example magnetite is a mineral of iron oxide (fe₂o₃).

calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (caco₃).

dolomite is an   carbonate mineral composed of calcium magnesium carbonate (camg(co₃)₂).