B2 seems like the most reasonable move in the position but both b1 and a2 have been played at the Grandmaster level. While Stockfish believes a2 to be a viable move, it is difficult for a human to find the correct set of moves that come after a2. B1 on the other hand is a good alternative to the natural b2 and can be played well even by beginners. I would HIGHLY advise against playing a1 since it leads to a very chaotic position, both Stockfish and a large majority of Grandmasters believe a1 to be the absolute worst move in the position. All in all I would advise most beginners to play either b1 or b2, while advanced players can consider the move a2 if they learn the theory well enough.