Student or Learner
I think "aloud" is same as "loudly", but "loudly" is the correct answer. Isn't it?
19)A few years ago, [while / during] setting up camp deep in the rain forest, Morgan and Sanz heard a party of chimpanzees [vocalizing / to vocalize] [loudly / aloud] in the distance. They thought the chimpanzees were moving rapidly among trees. The chimpanzees were yelling louder and they seemed to be heading straight for the camp. Morgan and Sanz felt the chimpanzees would soon be [near / nearly] o
"Aloud" doesn't mean the same as "loudly". It simply means audibly. If I read a book in bed at night, I read silently. I read in my head. If I read a book to a small child, I read the words aloud (also "out loud"). I wouldn't read a book to a child loudly, especially if I wanted them to go to sleep as a result of my reading to them. I would give a speech by speaking aloud to a group of people. I would only need to speak loudly if there were some people there who couldn't hear me if I spoke in my normal speaking voice.
However, in the piece you posted, I agree that it's more likely that the chimpanzees were heard vocalising loudly. Merely vocalising probably wouldn't be something worth commenting on. In addition, if you vocalise something, it means you say/do it aloud. "Vocalising aloud" would be tautologous.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.