Six soldiers were concerned ____ the plot.

Concerned for/about - a worried or nervous feeling about something.
বাংলা অর্থ = কোন কিছু সম্পর্কে উদ্বিগ্ন হওয়া।
Example - I'm a bit concerned about/for your health.

Concerned in = involved n something.
বাংলা অর্থঃ কোন কিছুতে জড়িত থাকা।
Nine men were concerned in the plot.
অর্থাৎ, নয় জন লোক গোপন পরিকল্পনা বা ষড়যন্ত্রটিতে জড়িত ছিল।

Concerned with = interested in, in a context.
বাংলা অর্থঃ আগ্রহী/কোন বিষয়ে
Example - She was more concerned with flirting than with getting the job done. Today's lesson is concerned with punctuation.

Source: Longman Dictionary of Contemporary English, Cambridge and Merriam-Webster Dictionary.