The correct answer is Option 4 i.e I thought I had better did something about [ alt="428" src="//storage.googleapis.com/tb-img/production/20/09/428.png" style="width: 40px; height: 40px; float: left;">Explanation: Reading the above sentence we find that: 'Had better' and 'Had sooner' are followed by the V1 form of the verb. Hence here did changes to do following the above rule. Example: They had better died than surrender. (wrong) They had better die than surrender. (right) Hence from the above points, we find that the error lies in had better did which is Option 4 and therefore the correct answer. The correct sentence is: 'I thought I had better do something about it now before it really gets going, because I would not want something like this on my own doorstep.'