The summary must include the main points of the paragraph. It need not include illustrations of the point or elaborations of it but it MUST always include the main point
Do not get distracted by less important facts in the paragraph. The first thing you must search among the options is whether or not they address the main point of the paragraph
A summary should be crisp and to the point. Hence, eliminate options that give too much of irrelevant details
How to prepare for Summary questions:
The preparation for summary questions will be the same as for all reading comprehension type question - you need to learn to grasp the gist of the paragraph
To be able to cut to the main point of the paragraph, practice reading and summarizing paragraphs. While summarizing the paragraph try to identify what was the most important point or points of the paragraph. The whole passage would revolve around this point. If an example or illustration is given, it would be an illustration of this main point.
Read a lot newspapers, particularly editorials, and try to identify the main point of the editorial, the key arguments forwarded and the conclusion of the editorial
If the missing line is the last line of the paragraph, it should complete the chain of thoughts that are being expressed in the paragraph
The last line need not necessarily be a summary of the paragraph but it must necessarily follow the line of thought being expressed in the paragraph and complete that chain of thought
While solving para completion questions remember to find the chain of thoughts running through the paragraph. Two common constructions of paragraphs are firstly, starting with an analogy or illustration and then tying in with the general concept and secondly, introducing the general concept and then forwarding arguments or illustrations in favor of the same. Hence, the last line of the paragraph must always state some conclusion about the general concept - a conclusion that follows either from the illustration/analogy or from the arguments made in favour of the concept.
If the missing line is not the last line - find the option that connects the thought being expressed just before and just after the missing line
How to prepare for Para Completion questions:
Similar to all reading comprehension questions, read as many good publications as possible
Try to identify the line of thought running through paragraphs. Hence, you should be able to identify introduction to concept, example/illustration, arguments made in favour and against and the conclusion drawn from the arguments.