Final answer:
The novel does not clearly state the fate of the letter Manning mails for Solomon Northup in 'Twelve Years a Slave,' and the information provided does not cover this event, so we cannot confidently select an answer regarding its fate.
Step-by-step explanation:
In Twelve Years a Slave, Solomon Northup recounts his harrowing experience of being kidnapped and sold into slavery. Regarding the letter Manning mails to Northup, the novel does not provide a detailed account of the fate of the letter. Due to the nature of communication and the perilous situation for slaves and their allies at the time, one could infer that Northup's attempts at sending letters likely faced numerous obstacles, such as the possibility of being intercepted and destroyed or lost in transit.
Direct evidence from the book would be required to say definitively what happened to the letter, and since the provided references do not explicitly cover the event of the mailing of a letter by Manning, we cannot confidently select from the options provided.