Select the phrase/connector (in between I and I

(b) We use 'as soon as' to show that 'something happens immediately', i.e. 'at the very moment another action is completed', or 'shortly after another action is completed'. This case perfectly fits in the context of the two statements given as the speaker realised they had forgotten their wallet [statement I] when they reached the hostel gate [statement II]. Thus, the connector/linking phrase in (ii) can be correctly used to form a single sentence. 'Secondly' is used to establish a concept/thing/idea at the second position when describing a sequence of things, while 'including' is used to give an example, neither of which is the case here, and thus, they are eliminated. The sentence so formed is as follows: As soon as I reached the hostel gate, I realised I had forgotten my wallet at the mall. Hence, the correct answer is option (b).