The correct answer is A. Lacing.
Lacing is a method of providing lateral stability to a built-up steel column by connecting the individual members together with steel straps or rods. This helps to prevent the column from buckling under load.
Battening is a similar method, but it uses horizontal members to connect the individual members of the column. This helps to prevent the column from twisting under load.
C. Tie plates are used to connect the individual members of a built-up steel beam. They do not provide lateral stability to the column.
D. Perforated cover plates are used to protect the steel members of a built-up steel column from fire and corrosion. They do not provide lateral stability to the column.
Lacing is the best arrangement to provide unified behavior in built up steel columns because it provides both lateral stability and torsional resistance. Battening and tie plates only provide lateral stability, while perforated cover plates do not provide any structural support.