Hi. Thank you. Do you see any differences for the following (with the definite article and without)? I am sorry for not providing the whole sentence for the first sentence.
1. ... thought to have been caused by (the?) contamination of the land.
2. Please see this as (the?) fulfillment of your dream.
My comment on your 3 sentences remains the same as above: There is no difference unless context reveals that the retention and research are mentioned previously or otherwise specified.
Superlatives by nature take normally take the definite article; it is required in your sentence.
To me, the article is optional with numbers and letters.