Small tables with only a handful of rows can still benefit from being indexed. Queries to find a single row will still perform better when a non-unique index is used over full-scanning the whole of the table. In this quiz, the FTS uses 7 consistent gets (logical IOs), but the query with a normal index uses only 3 (choice 2).
It is good practice to define primary keys on all your tables however, so these tables should have a primary key. These are backed by an automatically generated unique index (though you can specify an existing index if you wish), which Oracle can then use when executing queries. So in this quiz, both choices 3 and 4 both use "index unique scans" in the execution plan, consuming 2 consistent gets.
Query performance can be improved further by building the table as an index-organised table. This means that instead of having two structures (a table and an index), there is only one - the index! This can reduce queries against these small IOTs to a single consistent get as in this quiz. The benefits may be small, but these can add up if the table is frequently queried in a busy system.