I would say that the answer to your question would be that the subject(s) of the sentence is/are the following one/s: Tablet, pens. That is, the correct option would be B.
In this sentence, the PP indicating the location of the tablet of paper and the pens appears in initial position in the sentence. The sentence sounds odd and is not grammatically correct because in these cases an existential there is introduced. Take a,
a) In the desk drawer, there are a tablet of paper and some pens
b) *In the desk drawer, are a tablet of paper and some pens
The NP "a tablet of paper and some pens" would function as the subject complement and the existential there acts as grammatical subject filler(a)
