Answer:
The first two options are correct; Ten billion & 10 billion.
Explanation:
When we are writing we want it be as comprehensible as possible. Adding information that is hard to read can be distracting and the audience can lose interest. For example take a look at the three sentences below:
<em>Maria thinks that the world population is ten billion.</em>
<em>Maria thinks that the world population is 10 billion.</em>
<em>Maria thinks that the world population is 10,000,000,000.</em>
The first two sentences are easily readable, but in the third sentence the reader starts counting zeroes, and therefore is distracted from the main subject and he may even decide to not pursue it further.
Therefore the first two are best options when writing a large number in a sentence.
