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
Tip
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.
