Which of the following is not an element but a compound?

Which of the following is neither an element nor a compound?

Which of the following is not an "element + element to compound " type reaction?

Which of the following pair of elements form a compound with maximum ionic character ?

Which of the following elements in an organic compound cannot be detected by Lassaigne’s

Which of the following elements will form a compound with oxygen in which one atom of the element is combined with one atom of oxygen to form an oxide of formula MO ?

Which of the following element form interstitial compounds?