( b) is normal colloquial English ( a) is old-fashioned. In everyday American English, it would sound prudish.
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sb70012He put the things in his bag in case he ……… them.None of the answers sound satisfactory to me, but I would have chosen c).
ozzourtiHe put the things in his bag lest he (should) forget them.Argh! I knew someone would eventually mention the elephant in the room.