Yes, B is unreserved.
For future reference, on a 4 carriage train, the unreserved Standard Class coach is B, and:—
4-car – B
5-car – B and F
8-car – F and L
9-car – B, F and L
10-car – B, F, H, and L
(For completeness, First Class is Coach A on 4- and 5-car trains, and Coaches A and G on 8-, 9- and 10-car trains, and it always has some seats kept unreserved.)