Which 2 diagrams show suitable methods for investigating the speed of the reaction

I would say D: 2 and 4.

The first line of the question is very important. X reacts with Y to form a gas.

For 2, we let the gas escape and measure the change in mass during the reaction.

For 4, we capture the change in volume of gas during the reaction.

1 does not work: some of the gas escapes through the cotton and the gas captured is not recorded correctly.

3 does not work: the gas is trapped and the mass balance does not record a change in mass.

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.