Question

it is helpful to do sth it is helpful for doing sth it is he

Answer 1

It's helpful to do sth 是正确的.It 是形式主语 真正的主语用 to do sth

It is helpful to be informed .

be helpful to sth to 是介词 ,后应该跟 动词的ing 形式所以A BC都不对

be helpful in （doing ）sth 表示在某方面有帮助

Doing exercise is helpful ___in keeping_____ healthy