Over time, smart phones have become .
smaller and harder to use
more powerful but harder to use
less expensive for a certain amount of computing power but easier to use
larger and easier to use

C. Less expensive for a certain amount of computing power.

Explanation:

Overtime, smartphones have become less expensive for a certain amount of computing power but easier to use.

Smartphones are more than just a way to reach out to people, they have a lot of features aside from calling and texting.

Basically, smartphones have high-speed internet accessibility, high definition digital cameras, games, text-to-speech app, GPS and a variety of fun social media applications for the Apple iOS and Android operating system.

Hence, with the above-mentioned features it performs all sort of tasks seamlessly to the satisfaction of the users with easy commands.