That is a
of a question.
The reference document is ICAO Annex 3 Appendix 3.
Based on the information provided the best answer is 6000m which is the prevailing visibility until 1900. The code FM1900 indicates the visibility will start to deteriorate at 1900 and in this case because there is no TL code it means the change to 2000m will be complete at the end of the trend period (by definition, 2 hrs from the time of the METAR).
For the answer to be 2000, the trend would have to be coded either BECMG TL1900 2000, or BECMG AT1900 2000, or BECMG FM(a time) TL1900 2000.
So in this case the visibility is 6000m until 1900 when the deterioration to 2000m will commence.