The correct option is C Be in charge of
In "Army personnel man the border", 'man' is a verb in this context here, the verb means 'to operate or defend'.
Option C: This provides a similar meaning of 'man' in this sentence. Hence Option C is correct.
Options A, B and D are all definitions of 'man' as a noun. Each word refers to a person, whereas 'man' functions as a verb, it refers to an action.
The meaning of a word differs according to its function.
Hence options A, B and D are incorrect.