Which of these elements has an electron configuration of [Kr]5s2?

barium

calcium

rubidium

strontium

Given this abbreviated electron configuration, we begin from the row of the periodic table that contains Krypton (Kr). Then since the remaining electrons are 5s2 (which indicates 2 electrons in the 5s orbital), we check the next row (the 5th row) and find the 2nd element, which would contain two electrons. The first element is rubidium, while the second is strontium, so this means that Sr is the final answer.

Which of these elements has an electron configuration of [Kr]5s2?

The element that has an electron configuration of [Kr]5s2 is strontium. The rest of the given choices do not answer the question above.