jogging is one of the activities ___________________ him lose weight.
Which one should I use - "which helps" or "which help." - and why?
jogging is one of the activities ___________________ him lose weight.
Which one should I use - "which helps" or "which help." - and why?
As "activities" is plural it needs "which help", which is the plural form. The sentence would always be taken as meaning that there are several activities which help him lose weight, and jogging is one of them. To say that jogging is one of several activities, and it is the one which helps him lose weight you could use "jogging, which helps him lose weight, is one of the activities". Alternatively you could split the sentence to "jogging is one of the activities. It helps him lose weight".
Singular and plural are both correct. It depends of the context:
If the activities refer to chess, reading and jogging, then one single activity (jogging) is the subject, giving which helps.
If activities were jogging, basket-ball and swimming, the 3 sports are the subject, what gives which help.