The correct choice is (c) Open tendering
The best I can explain: On larger projects, there may then be a pre-qualification process that produces a short-list of suitable suppliers who will be invited to prepare tenders. This sort of pre-qualification process is not the same as selective tendering.
Open tendering has been criticised for attracting tenders / expressions of interest from large numbers of suppliers, some of whom may be entirely unsuitable for the contract and as a result it can waste a great deal of time, effort and money. However, open tendering offers the greatest competition and has the advantage of allowing new or emerging suppliers to try to secure work.