@JohnScott The only way that answer (A) would strengthen the argument is if you misunderstand the argument's conclusion. If you understand the argument to mean "lichen and grass were the only burning resources to Neanderthals at the time," then (A) weakens the idea and (B) strengthens it.

However, that's not what the stimulus is arguing. Whether lichen and grass were the only resources they could use for smoking is not a matter of question. We are simply, as an inarguable fact, told that lichen and grass (which produce more smoke) were found in Neanderthal fireplaces.

But what does that mean? The stimulus hypothesizes that it means that they used those materials for preservation. This is what we're tasked with weakening: the idea that these resources were definitely used to preserve meat by smoking.

The fact that there were other fireplaces that produced more heat than smoke doesn't weaken the possibility that lichen and grass was indeed used to preserve meat. AKA having other methods of cooking/preservation doesn't deny the conclusion's possibility.

What WOULD weaken this idea is (B), which attacks the supposed intention of the lichen and grass. The argument supposes that it was used for preserving meat. But what if it's actually used for ceremonial, communicative, or protective purposes (as JY suggests)? Or, what if it was all they had? Meaning, their intent wasn't necessarily to smoke meat for preservation.