[betterunix2]

100 shares is considered a "lot" and is a standard size. True, an investor can own fewer than 100 shares, but every so often a company will make an "odd lot offer" forcing investors to either sell their shares or buy enough to hold at least 100.

Another issue is that out of the money contracts for a single share would frequently trade for pennies, which would greatly increase transaction costs and make the market less efficient. So you would have to standardize on some amount that is large enough to avoid such inefficiency, and 100 is a convenient number for mental arithmetic and reduces confusion. In theory you could have standardized on a larger number like 1000, which is equally convenient, but that would have priced out retail investors who may not own 1000 shares of a company whose share price is in the double or triple digits (which is common) and reduced the liquidity of the options market (which is already not very liquid).