When the buyer of a call option on a stock exercises the option, the seller of the call option most likely must:

When the buyer of a call option exercises the option, they have the right to buy the underlying stock at a predetermined price, known as the strike price. The seller of the call option, often referred to as the writer, is obliged to fulfill this contract. Therefore, the seller must deliver the underlying stock to the buyer of the call option at the strike price.

This obligation to deliver stock arises from the nature of call options. The buyer exercises their right to purchase the stock, and as part of the option contract's terms, the seller cannot refuse this obligation. The transaction results in the transfer of the stock from the seller to the buyer, with the seller receiving the strike price in exchange for the shares.

The other options do not accurately reflect the mechanics of a call option exercise. The seller is not taking delivery of stock, delivering cash value, or purchasing additional stock; instead, their role is to provide the actual shares to the buyer as specified in the contract.