Suppose one question posting contains several question. If I can answer one question, but cannot answer all of the questions, is it ok to post my answer as an answer, and not just as a comment?
Yes, partial answers, given as actual answers, are acceptable and even desirable.
If OP of a question in some case should feel they prefer the remaining question as an unanswered question, they could in turn ask a new question, focusing on what is still open and what they have learned from the partial answer.
Of course, both should be done using common sense. For example, the partial answer should be still substantive, and a reasking of the remaining question should be done only (if at all) with some significant delay and clear indication of the earlier question.
5$\begingroup$ Instead of reasking the remaining question in case of a partial answer, one can also edit the original question and state which parts have been answered by the answers already given, and which parts are still open. $\endgroup$– Stefan Kohl ModNov 12, 2014 at 11:46
$\begingroup$ Good point. Editing is in general preferable. However, there are precedences of questioners (or other interested parties) expressing some discontent that some question is no more in the "unanswered" list while clearly not having been answered. My remark is meant to address these (rare) cases where in earlier discussion the practice I propose was recommended or at least declared admissible, IIRC. $\endgroup$– user9072Nov 12, 2014 at 12:33