I think you would simply just be lucky if you timed your option purchase to coincide with a Roche buy out. What would happen depends entirely on which options you pick, keep in mind that option value deteriorates with time. But here is a for example; You buy $12 strike price January call options and they cost $1. Roche buys IMGN for $16 before these expire, so your $12 option would be $4 in the money, so you might reasonably expect them to be worth $4 and some change.
If you are that bullish you can also sell puts against the stock. The risk you run here is that no buy out comes and the stock is below the strike price of the puts you sold, thus you would be required to purchase the stock, i.e. the option holder would put the stock to you. If the stock trades sideways or a buyout does come, you keep all the money from the sold puts. So no upside to the buyout from Roche if it were to happen.
Everyone and their hamster knows that TDM1 is going to be approved, just when it happens no one knows. So TDM1 approval is likely going to be a non-event for the stock price. Why has Roche not purchased IMGN by now? They better than anyone know how valuable TDM1 is to them, so I can only assume that they are OK with just paying the small royalty to IMGN and are not interested in buying them. Why people think that TDM1 approval somehow triggers this or takes all the risk out of buying IMGN just doesn't make sense to me.
It would be far better donate to a good organization like Samaritan's Purse than to waste them on buying IMGN options at this point. At lease you would be doing some good and get a tax break. All long IMGN calls eventually go to zero!