What determines whether fructose is D or L?

Fructose is a ketohexose. In the Fischer projection of a monosaccharide, the D/L configuration is decided by the configuration at the chiral carbon farthest from the carbonyl group.

For fructose, the carbonyl group is at $C_2$, so the farthest chiral carbon is $C_5$.

• If the $-OH$ on $C_5$ is on the right, it is D-fructose.
• If the $-OH$ on $C_5$ is on the left, it is L-fructose.

Structure of fructose in Fischer projection

Fructose has the chain: $CH_2OH-CO-(CHOH)_3-CH_2OH$

The standard Fischer projection of D-fructose has the following $OH$ arrangement:

• $C_3$: left
• $C_4$: right
• $C_5$: right

Therefore, L-fructose is its mirror image:

• $C_3$: right
• $C_4$: left
• $C_5$: left

How to identify the correct option

So the correct Fischer projection for L-fructose must satisfy all of these:

• top carbon: $CH_2OH$
• second carbon: ketone group $C=O$
• three chiral centers below it
• $OH$ positions at $C_3, C_4, C_5$ as: $\text{right, left, left}$

Matching with the given options

Among the provided options, the one corresponding to this mirror-image arrangement is Option B.

Conclusion

Hence, the correct structure of L-fructose is: $\boxed{\text{B}}$