Hello, people, no worries when it comes to this one. To begin with, let's get the concepts down. Things need to be compared to the correct thing. An apple can be compared with an orange, but not with a car. A car can be compared with a truck, but not with a tornado. (All things being equal that is!)

In any event, the sentence here is clearly comparing children with adults, so keep those two as your anchor.

i]Children's tendency to be more susceptible to bronchitis and asthma than adults is[/i] possibly because, according to recent research, children retain 35 percent more of inhaled airborne particles on the surface of their lungs than adults do.

(A) Children's tendency to be more susceptible to bronchitis and asthma than adults is

Before you even fully grasp the whole meaning of this sentence and all the nuances, focus on WHAT is being compared to WHAT. Here children's tendency is compared to adults. Sorry, that is an immediate elimination.

(B) Children's tendency to be more susceptible than is the case with adults to bronchitis and asthma is

Again this is another tendency vs adults comparison. Another freebie elimination

(C) Children tend toward being more susceptible to bronchitis and asthma than adults, which is

When you see a which in the sentence immediately check the logical structure. Here which refers to adults, the preceding noun. This makes for a completely nonsensical meaning for what follows. But even if you did not check all that meaning business, what should strike you is that adults is plural and what follows which is the singular is. Eliminate. There are other issues afoot here but look for the easy elimination.

(D) Children, who tend to be more susceptible to bronchitis and asthma than are adults, are so

OK, I get it, you immediately see the Children and Adults and you think this could be a contender. Remember, the portion inside the commas can be deleted from the sentence. Indeed, I think in this case not only should it be something you can delete, but the overall meaning of the sentence should also still make sense and being conveyed properly. Here's what happens if you delete the portion in the middle: Children are so possibly because.... yuck! Eliminate. Clearly the portion inside the commas is awkward as well but look for the easy elimination and move on. Time, people, is the enemy.

(E) Children tend to be more susceptible than adults to bronchitis and asthma

Bingo and we're done. Children compared to adults. Clear and concise.