Which of the following universal forces is the weakest?

A) electric

B) gravitational

C) magnetic

D) strong nuclear

I also need some explanation for this one

Well, electric and magnetic are actually the same in nature (as basic force in the Universe). The four, in decreasing order of strength, are:

Strong nuclear, Weak nuclear, Electric/Magnetic (in roughly the same range) and Gravitational (much weaker than Electric/Magnetic - - a magnet less than 1 kg beats Earth which is quite a lot of gigatons, roughly)