Join the following sentences to make one complete sentence without using and, but or so.
My grandfather is very old. He is very active.
Topic: Synthesis — clause of contrast
Answer
Although my grandfather is very old, he is very active. (Also acceptable: In spite of being very old, my grandfather is very active.)
The relationship is CONTRAST — age would lead you to expect inactivity, and the second sentence defeats that expectation — so ALTHOUGH or THOUGH is correct.
The phrase version, "In spite of being very old", avoids a conjunction altogether, which is what the instruction asks for. Note the forms: although / though + full clause (he is old) in spite of / despite + noun or -ing form (being old)
THE ERROR TO AVOID: never use "although" and "but" together. "Although he is old, BUT he is active" is wrong — one conjunction is enough.
THE METHOD for every synthesis question: identify the relationship first — contrast, reason, time, condition — and the conjunction follows.