The force in AC of the truss shown in the given figure, is

ANS:B - 4t tension

There is only one horizontal force 4T acting at the top. This shall pass undiminished to the bottom as it is a pin joint frame. So horizontal reactions at A & B should be 4T each.

Respectively, But, notice that B is roller end so horizontal reaction at B=0 and free to move towards right under the action of the force. While horizontal reaction at A remains 4T towards LHS. Thus, while at A it remains fixed, but at B it moves towards the right. Thus creating an extension force on the member AB. So the member AB is under 4T tension.