The correct answer is $\boxed{\text{B. 1.3 L}}$.
The effective length of a battened strut effectively held in position at both ends but not restrained in direction is taken as 1.3 times the actual length of the strut. This is because the battens provide lateral support to the strut, which reduces the effective length of the strut and increases its load-carrying capacity.
Option A is incorrect because the effective length of a strut is always less than or equal to its actual length. Option C is incorrect because the effective length of a strut is always greater than or equal to 1.1 times its actual length. Option D is incorrect because the effective length of a strut is always greater than or equal to 1.5 times its actual length.