Not all call options that have value at expiration, exercise by purchasing the security (or attempting to, with funds in your account). On ETNs, they often (always?) settle in cash. As an example of an option I'm currently looking at, AVSPY, it settles in cash (please confirm by reading the documentation on this set of options at http://www.nasdaqomxtrader.com/Micro.aspx?id=Alpha, but it is an example of this). There's nothing it can settle into (as you can't purchase the AVSPY index, only options on it). You may quickly look (wikipedia) at the difference between "American Style" options and "European Style" options, for more understanding here.
Interestingly I just spoke to my broker about this subject for a trade execution. Before I go into that, let me also quickly refer to Joe's answer: what you buy, you can sell. That's one of the jobs of a market maker, to provide liquidity in a market. So, when you buy a stock, you can sell it. When you buy an option, you can sell it. That's at any time before expiration (although how close you do it before the closing bell on expiration Friday/Saturday is your discretion). When a market maker lists an option price, they list a bid and an ask. If you are willing to sell at the bid price, they need to purchase it (generally speaking). That's why they put a spread between the bid and ask price, but that's another topic not related to your question -- just note the point of them buying at the bid price, and selling at the ask price -- that's what they're saying they'll do.
Now, one major difference with options vs. stocks is that options are contracts. So, therefore, we can note just as easily that YOU can sell the option on something (particularly if you own either the underlying, or an option deeper in the money). If you own the underlying instrument/stock, and you sell a CALL option on it, this is a strategy typically referred to as a covered call, considered a "risk reduction" strategy. You forfeit (potential) gains on the upside, for money you receive in selling the option.
The point of this discussion is, is simply: what one buys one can sell; what one sells one can buy -- that's how a "market" is supposed to work. And also, not to think that making money in options is buying first, then selling. It may be selling, and either buying back or ideally that option expiring worthless.
Now, a final example. Let's say you buy a deep in the money call on a stock trading at $150, and you own the $100 calls. At expiration, these have a value of $50. But let's say, you don't have any money in your account, to take ownership of the underlying security (you have to come up with the additional $100 per share you are missing). In that case, need to call your broker and see how they handle it, and it will depend on the type of account you have (e.g. margin or not, IRA, etc). Generally speaking though, the "margin department" makes these decisions, and they look through folks that have options on things that have value, and are expiring, and whether they have the funds in their account to absorb the security they are going to need to own. Exchange-wise, options that have value at expiration, are exercised. But what if the person who has the option, doesn't have the funds to own the whole stock?
Well, ideally on Monday they'll buy all the shares with the options you have at the current price, and immediately liquidate the amount you can't afford to own, but they don't have to. I'm mentioning this detail so that it helps you see what's going or needs to go on with exchanges and brokerages and individuals, so you have a broader picture.