se douter que expresses expectation, feeling rather than doubt. This is a hypothetical tone and requires the use of conditional expressions. Choosing A

douter que is also quite special,

(1) [The clause generally uses the subjunctive form plus ne; if it uses the indicative form or the subjunctive form without adding ne, the tone will be Expressing absolute certainty]:

Je ne doute pas qu'il ne vienne. I believe he will come. / Je ne doute pas qu'il vienne (vient)I believe he will come. The tone is more certain than the former.

(2) If the clause is in a hypothetical mood, it generally uses a conditional expression]:

Je ne doute pas qu'il accepterait, si j'insistais. I believe that as long as I persist, he will will accept it.

If you don’t understand, you are welcome to ask. I hope it helps. Thank you.