A) tendency is susceptible to bronchitis and asthma--> illogical: tendency cannot be susceptible to some disease

Children's tendency to be more susceptible to bronchitis and asthma than adults
illogical comparison: tendency compared with adults.


is/are because
is/are when
is/are where
all are incorrect.
There is a situation in which is because can be correct but that situation has no precedence in official material.

B) tendency is susceptible--> illogical as is the case in A.
is because--> incorrect, as is the case in A.

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

which clause has no eligible referent.
is because--> incorrect, as is the case in A and B.

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

non essential modifier is downright illogical for two reasons:
D says that all children tend to be more susceptible to bronchitis and asthma than are adults--> illogical.

Secondly, even after omitting non essential modifier, the meaning should be sensical: not the case here.
Children are so because...