Hola William

You could use "van a llegar" here, but the reason why is because the beginning of the sentence is actually the part that is already conveying the "probability/conjecture" -> "Me pregunto...".

Maybe it is not a good idea to include that option within the possible answers.

For other sentences you would have to use the simple future "llegarán" to express the probability/conjecture about the future. For example:

Estoy muy ocupada así que llegaré sobre las doce o doce y media.

I am very busy so I might arrive at about twelve or twelve thirty.

If you were to use "voy a llegar" here, it would sound very definite, as if you knew for sure you are going to arrive at that time.

Hope this clarifies it.

Saludos

Inma