Today's multiple choice question: Who Is Moon Reagan? a.)He holds every Southwest Conference Record for the discus and the javelin and is now organizing a hostile takeover of both Wendy's hamburgers and General Dynamics. b.)He is the lieutenant governor of a Rocky Mountain state, where the governor of the opposite political party is the target of a grand jury investigation about the illegal duplication of cassettes recorded by Madonna. c.)He is the founder of the successful cable channel that broadcasts live all hearings and meetings of the National Oceanic and Atmospheric Administration as well as the card catalogues of the Library of Congress. d.)None of the above.

It may well be the highest compliment to be paid to Ronald Reagan's older (by 21/2 years) brother that a lot of us still are not sure just who Neil "Moon" Reagan is. This stands in dramatic and welcome contrast to previous presidential brothers. When their siblings were president, very few people had to ask who Donald Nixon, Sam Johnson and Billy Carter were. That was because all three were notorious for their antics, which included obtaining large unsecured loans from Howard Hughes and generous allowances from unfriendly Middle Eastern governments.

Presidential siblings and offspring have frequently traded on their relatives' influence to promote their business schemes, and worse. But Neil Reagan, to the regret of the supermarket tabloids, has never been a limited partner of Robert Vesco's or double-dated with Claus von Bulow. Americans have no idea of what Neil Reagan thinks about either aid to the contras or his niece Maureen's husband. Just two more reasons why we ought to keep Neil Reagan in the position permanently.

In addition to being a terrific First Brother, Neil Reagan is apparently a pretty good only brother too. In 1962, when 51-year-old Ronald was unemployed, Neil, then a successful advertising man, persuaded him to accept the job of host in the TV series "Death Valley Days," which was sponsored by one of Neil's clients, Boraxo. But brotherhood had not been a one- way street between the Reagan boys. Years earlier Ron had convinced Neil to follow him to Eureka College, where they were football teammates.

By his own count, Neil Reagan stayed overnight once in Sacramento during the eight years his brother was California governor. That once, he has explained, was because the "danged fool got himself sworn in at midnight." He has been in Washington for both inaugurals but has stayed quietly away the rest of the time. Now in the summer of 1985, when wealthy clients faced with the elimination of some cherished tax preferences seem to be handing out six-figure retainers to anyone who knows the cab route from Capitol Hill to the White House, Neil Reagan -- resisting any and all overtures -- represents no client before the House Ways and Means Committee or anyplace else.

In the final analysis it is what Neil Reagan hasn't done that makes him a credit to his family and his country. Thanks to Neil Reagan, we have no nasty anecdotes about Ronald Reagan as a mean kid and no complaints about "Mom likes Ronnie best."

The 1988 presidential candidate who announces first that, if elected, he will ask Neil Reagan to remain as First Brother will show the kind of judgment that could be a big boost in the New Hampshire primary. He's experienced, diplomatic and discreet; let's keep Neil Reagan as First Brother. It may be the highest compliment to Ronald Reagan's brother that a lot of us are not sure who Neil Reagan is.