Answer:
b. 375 N
Explanation:
System of forces in balance
ΣFy = 0 Equation (1)
Forces acting on the board
T₁: Tension in the  left chain ,  vertical and upward
T₂ = 250 N : Tension in the right chain ,  vertical and upward
W₁ = 125 N : Weight of the  board , vertical and downward
W₂ =  500 N : Weight of the  person , vertical and downward
Calculation of the T₁
We apply the Equation (1)
ΣFy = 0  
T₁+T₂-W₁-W₂ = 0
T₁ = -T₂+W₁+W₂ 
T₁ = -250 N+ 125 N+ 500 N
T₁ = 375 N