A. The availed themselves of the train
B. The availed them of the train
C. If one want to leave a message , I will be happy to take them
D. Thousand of people were present
Explanation: RD(I)-72; কিছু word (যেমন: avail, exert, absent etc.) যাদের পরে pronoun আসলে তা অবশ্যই reflexive pronoun হয়।