People trying to reduce their weight should not only write down what and when they eat or drink each day, but also well.
a. who they eat with
b. why they are eating
c. where they are eating
d. how much they weigh

I'd say the most correct answer here is D. How much they weigh. 
Writing doing what and when they're eating/ drinking is great and all, but if you don't know which foods are actually helping you lose weight then it's useless. That's why I think recording your weight at the beginning of the day at certain intervals (once a week, every other week, whatever) is just as important at recording what you're eating.