Many people believe that such activities as owning a pet, taking care of another's pet, and, of course, babysitting an infant not only confer the capacity of friendship on a child but also develop role-taking skills, since that child must put him- or herself in the pet or infant's situation and imagine how that pet or infant feels.
not only confer the capacity of friendship on a child
but also develop --> uses the correct Idiom: "Not only..But also" --> hence is correct.
confer the capacity of friendship on a child but also are developing--> Should use the correct Idiom: "Not only..But also" --> hence is not correct.
are conferring the capacity of friendship on a child
but also are developing--> Should use the correct Idiom: "Not only..But also" --> hence is not correct.
not only confer the capacity of friendship on a child, they are also developing--> Should use the correct Idiom: "Not only..But also" --> hence is not correct.
are conferring the capacity of friendship on a child, and they are also developing --> Should use the correct Idiom: "Not only..But also" --> hence is not correct.