From the Official Rules of Monopoly, "... he must turn over to that player all that he has of value and retire from the game." A bankrupt player cannot do anything at all including mortgage any properties. Some like to allow a player to trade after being declared bankrupt *if* they can get enough funds to payoff the debt but that is not allowed by the rules -- there are no provisions for coming out of bankruptcy. The moment a player is bankrupt there is nothing left for them to do.
Prior to throwing the dice a player could in theory use the tactic described to reduce their net value but we (the group of people I play with) expect every player to always improve their overall position with every action taken. Likewise, prior to throwing the dice, a smart player could make trades to improve their cash position enough in order to avoid bankruptcy; again with the same expectation.
If a player takes an unreasonable (in the sense that they aren't improving their overall position) action we won't play with them again in the future. A trade can obviously be deemed reasonable even if the player ends up with less cash. A trade can be deemed reasonable even if the player ends up with less net worth (in fact, this is almost always the case for one of the traders (exactly even trades are unlikely, unnecessary, and could even be deemed unreasonable)). A trader can obviously over-value a trade out of a simple lack of experience and this is not necessarily deemed unreasonable. If the trader is honestly attempting to improve their overall position but just makes an error in judgment then that is not deemed unreasonable although it would be good to help the player learn and improve later. Taking advantage of an inexperienced player is mean spirited. It is possible for a trader to improve their net worth yet still be deemed unreasonable, e.g. if they own the unimproved and not mortgaged orange monopoly and trade New York Avenue for $201. Obviously breaking the monopoly for a net worth gain of $1 is unreasonable.
There is no option C.