Secret ballot and RMS Resolution
Votes in leadership elections are kept secret even after the end of the voting period for obvious reasons: by knowing that the ballot is secret, voters can feel free to express their opinion as they see fit. This constitutional guarantee sadly does not apply to General Resolutions.
The GR currently being discussed, "Statement regarding Richard Stallman's readmission to the FSF board", is significantly more polarizing and controversial than any of the DPL elections the project has ever had. Due to various examples of attacks based on guilt by association fallacies  and similar that took place during the recent debate about RMS, voters may not be able to freely express their opinion on this GR if the tally sheet is made public at the end of the voting period. It is unfortunate that the consequences of simply expressing an opinion may include personal attacks, ostracism and even losing one's job, but we have seen enough evidence of that to know that it is the case. 
The nuances of why someone might decide against a collective endorsement of the statement by the Debian Project are entirely lost by looking at the tally sheet alone. Maybe you are terrified by mob justice. Perhaps you disagree with some of the most extreme opinions expressed in the appendix to the statement, including that using singular gender-neutral pronouns instead of "they" is enough to be found guilty of transphobia. You could even simply think that Debian as a Project should not participate in the current culture of fear and intimidation resembling McCarthyism more than the type of society we all aspire to live in.
Whatever the case may be, it is not safe to vote choice 3: "do not, as the project itself, sign any letter regarding rms" unless the privacy of voters is guaranteed.