Achemical reaction between iron atoms and oxygen molecules can only occur if

a) the particles are heated
b) the atmospheric pressure decreases
c) there is a catalyst present
d) there are effective collisions between the particle

explain why the other choices are not necessary for the reaction to take place

The answer is D) There are effective collisions between the particle[s]. Collisions between particles is what happens during a chemical reaction (in this case, the oxygen and iron particles collide).

Choice A (the particles are heated) is incorrect because heating particles will make the reaction faster. However, there must be a reaction there in the first place.

Choice B (the atmospheric pressure decreases) is incorrect because decreasing pressure will increase volume, which does not have anything to do with the speed of the reaction. Assuming you have learned about equilibrium in a chemical system/equation, you will know that changing the pressure will make the equation shift to the side with less moles of elements. However, the number of moles is not specified.

Choice C (there is a catalyst present) is incorrect because a catalyst is something that speeds up a reaction. Similarly to choice A, there is no reaction to begin with.

Hope this helps. :)