The correct answer is A. diffident.
A diffident person is lacking in self-confidence or assurance. They may be hesitant to speak up or take risks, and they may worry about what others think of them.
A timid person is also
lacking in self-confidence, but they may be more fearful or anxious than diffident people. They may avoid social situations or new experiences, and they may be easily intimidated.A reserved person is quiet and restrained. They may not be as outgoing or talkative as other people, but they are not necessarily lacking in self-confidence.
A shy person is also quiet and restrained, but they may be more uncomfortable in social situations than reserved people. They may blush easily, avoid eye contact, or speak in a soft voice.
In conclusion, the antonym of confident is diffident. A diffident person is lacking in self-confidence or assurance, while a confident person is sure of themselves and their abilities.