Have you to Singapore? * 4 points
be
been
being
went

Have you been to Singapore?

Explanation:

Have you be to Singapore, have you being to Singapore, have you went to Singapore are all the wrong tenses.

Have you been to Singapore is in the right time tense.

- "the basic difference between keywords and subjects is that subjects (or "subject headings") have been predetermined, while keywords have not. a keyword is an easy place to start because you can use any word that seems to fit your search; this is called "natural language" searching."