The main idea of this paragraph is the third option , "was marked by frequent and short-term climate shifts.". There is textual evidence to support his idea along the paragraph. For example where it says: cycles of intensely cold winters and easterly winds, then switched abruptly to years of heavy spring and early summer rains. There is also evidence in the following quote " Little Ice Age was an endless zigzag of climatic shifts, few lasting more than a quarter century." here the author is telling us that the changes took place rapidly, and the processes were fast changing.
Option 1 is not correct because throughout the text there is no example of any prolonged cooling period. On the contrary, there is evidence of abrupt temperature changes.
Option 2 in not correct either because even though the Cro-Magnon period is mentioned, it only refers to our supposed imaginary image of the "Ice Age". it is used to show opposition to what the text really is describing later on.
Option 4 is mentioned at some point in the passage, mainly in the introduction, but "The interactions between the atmosphere and the ocean" is not what the text is about. In consequence, it is not a correct answer either. It explains the possible origins of these interactions . The examples provided along the paragraph describe the changes in temperatures and their frequency, not the causes. So this option would also be wrong.