Marcus volunteers for an organization that cleans up the neighborhood park. each volunteer is assigned to a different section of the park to pick up trash and pull weeds.

which motivation for giving is described in this paragraph?

to gain recognition
to develop new skills
to support an institution
to give back to the community

It's very hard to identify what motivates anybody to do anything . . . EVEN if you ask the person !

Indeed, from the language of the question, it must be pointed out that no motivation is actually "described" there.  It's possible that you may not have copied the whole paragraph.

I think it's reasonable to conclude that Marcus may be motivated by any one, two or three of these:

-- to gain recognition

-- to support an institution

-- to give back to the community.

The only one of the choices that I would seriously doubt is the second one.  If Marcus is developing new skills by picking up trash and pulling weeds, then we'd have to say that EITHER he really really needs every skill he can get no matter what it is, or that he is to be commended for finally trying to make something of himself.  

because if the tool that you are using is not balanced it there would be false mass.