Join the following sentences to make one complete sentence without using and, but or so.
Sania pushed as hard as she could. The door would not open.
Topic: Synthesis — clause of contrast
Answer
Although Sania pushed as hard as she could, the door would not open. (Also acceptable: Though she pushed as hard as she could, the door would not open.)
The relationship is CONTRAST — she pushed hard AND YET the door stayed shut — so ALTHOUGH or THOUGH is the right conjunction.
Note that "and" would lose the contrast entirely, turning two related facts into a bare list, and the instruction forbids it in any case.
A common error worth avoiding: do NOT use "although" and "but" together. "Although she pushed hard, BUT the door would not open" is wrong — one or the other, never both. The same applies to "although... yet".
The concessive conjunctions: although, though, even though, while, whereas, despite, in spite of.