In the following question, out of the four alternatives, select the alternative which will improve the bracketed part of the sentence. In case no improvement is needed, select "no improvement". LPG quickly (gain) popularity as it was easy to use and did not tarnish vessels.?

Option 3 : gained

The correct answer is gained. 

Reading the sentence we find that: 

• The sentence is talking about something that happened in the near past.  
• Thus the sentence is in the simple past tense and should use an appropriate verb. 
• The verb gain should be used in the simple past tense in order to fit into the grammatical structure of the sentence.
• The past tense of the verb gain is gained. 
• Other given options are incorrect as option 1 is a continuous verb and option 2 is a verb denoting the future. 
• Thus the correct answer is gained. 

LPG quickly gained popularity as it was easy to use and did not tarnish vessels.

Thus, the correct answer is option 3.