Answer:
Letter A is the correct answer.
Explanation:
For this question, let's use the discard method.
Letter B contains very specific information about a model of smartphones, so it does not contain the general idea of the text.
Letter C talks about two specific companies, their top models and their prices; if the article is about smartphones in general, the information is not enough for the summary.
In the last option, the main idea refers to the stock market, so it is very limited to be put in the summary.
It is necessary to point out that the summary must contain the most important information and readers should be able to get it from the beginning. Letter A is at the beginning of the paragraph and it also contains introductory information, so it is the correct answer.