Partnership

D(a 's wife) can do a competing business as per law, because there is no restriction regarding same in Partnership Act, 1932.
Partnership Act provides only for Partners can not engage himself in business which is completing to business of Partnership firm, but it's also as per Partnership agreement.

So, D can be do business, if it's not specifically restricted in partnership deed.
but, usually it's not happen because partnership is based on mutual understanding and cooperation between Partners. Partner are not Suppose to do the things which can affect the firm's business adversely.