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.

Variables,control,conclusion and finally scientific theory