Here is an example of setting options via mqtt.
Click the injects and the options will change.
Hope it helps.
[{"id":"3def0d93.e3949a","type":"mqtt in","z":"c74669a0.6a34f8","name":"","topic":"mqtt/option/dorpdown","qos":"2","datatype":"json","broker":"d675b749.04b9c8","x":180,"y":2940,"wires":[["15356478.1211ac"]]},{"id":"15356478.1211ac","type":"change","z":"c74669a0.6a34f8","name":"","rules":[{"t":"move","p":"payload","pt":"msg","to":"options","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":370,"y":2920,"wires":[["e1165fd6.2733a8","118f1320.d242b5"]]},{"id":"e1165fd6.2733a8","type":"ui_dropdown","z":"c74669a0.6a34f8","name":"","label":"","tooltip":"","place":"Select option","group":"2d4fe667.28f8ba","order":17,"width":0,"height":0,"passthru":true,"multiple":false,"options":[{"label":"","value":"","type":"str"}],"payload":"","topic":"topic","topicType":"msg","x":530,"y":2960,"wires":[[]]},{"id":"118f1320.d242b5","type":"debug","z":"c74669a0.6a34f8","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"options","targetType":"msg","statusVal":"","statusType":"auto","x":620,"y":2920,"wires":[]},{"id":"e729e11b.0f2818","type":"inject","z":"c74669a0.6a34f8","name":"option set 1","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"mqtt/option/dorpdown","payload":"[{\"Choice 1\":\"1\"},{\"Choice 2\":\"2\"},{\"Choice 3\":\"3\"}]","payloadType":"json","x":240,"y":3080,"wires":[["ee46676d.617f6"]]},{"id":"89a676e0.fc37c8","type":"inject","z":"c74669a0.6a34f8","name":"option set 2","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"mqtt/option/dorpdown","payload":"[{\"Choice 4\":\"4\"},{\"Choice 5\":\"5\"},{\"Choice 6\":\"6\"}]","payloadType":"json","x":240,"y":3120,"wires":[["ee46676d.617f6"]]},{"id":"ee46676d.617f6","type":"mqtt out","z":"c74669a0.6a34f8","name":"","topic":"","qos":"","retain":"","broker":"d675b749.04b9c8","x":370,"y":3080,"wires":[]},{"id":"d675b749.04b9c8","type":"mqtt-broker","name":"Localhost","broker":"localhost","port":"1883","clientid":"","usetls":false,"keepalive":"60","cleansession":true,"birthTopic":"","birthQos":"0","birthPayload":"","closeTopic":"","closeQos":"0","closePayload":"","willTopic":"","willQos":"0","willPayload":""},{"id":"2d4fe667.28f8ba","type":"ui_group","name":"demo","tab":"1caa8458.b17814","order":1,"disp":true,"width":"12","collapse":false},{"id":"1caa8458.b17814","type":"ui_tab","name":"Demo","icon":"dashboard","disabled":false,"hidden":false}]
the payload of the mqtt message should have the format
[{"Choice 1":"1"},{"Choice 2":"2"},{"Choice 3":"3"}]
Or others as descrided in the nodes help info.