No: Because your "efforts" are plural, so also should be your "volumes".
No. Would regular efforts be required
Yes, using that as a noun or an adjective. Such as: "That which is truth is believable." "That car was parked in my driveway." There are more convoluted forms: "That all of our efforts failed is extremely disheartening."
Both are grammatically correct, but "He can avail himself of the benefits" is more formal and emphasizes that he is taking advantage of the benefits personally. "He can avail of the benefits" is more informal and simply states that he can make use of the benefits available to him.
I applaud your efforts!
The answer would be correct A+
Example sentence - His children were clearly ungrateful for the efforts he made for them.
I was awesomely rewarded for my efforts.
Example sentence - His efforts were for naught as he had been given the wrong map.
Example sentence - She won a silver medal for her efforts.
Example sentence - The prevention of flooding is well worth the efforts.
A sentence using appreciable is: Her efforts were most appreciable at school.
His efforts were in vain.She was a very vain person.