Mo2men wrote:Dear Mitch,

1- GMAC in its model answer says that "The pronoun that in A and B should be deleted, since the pronoun one is sufficient to introduce the modifier and the sentence is more fluid without that." is it always with 'one'? if yes, is there any other words in line with 'one??

A and B: one that they believe
Here, that adds no meaning and thus is best omitted, as in the OA:
OA: one they believe to be of a type previously unknown to science.
However, a far better reason to eliminate A and B is that they convey an illogical meaning, as discussed in my post above.

An OA with one that:
Being heavily committed to a course of action, especially one that has worked well in the past, is likely to make an executive miss signs of incipient trouble.
Here, it is not possible to omit that.
Thus, the usage of one that is warranted.

2-Is the verb 'believe that' is correct? Have you seen it compared to 'believe...to be...'?

C: a supernova explosion, one they believe that it is of a type
This structure is not viable because the pronoun in blue and the that-clause in red are both serving the SAME FUNCTION:
Each seems --confusingly -- to be a direct object for believe.
Question: WHAT do astronomers believe?
Answer 1: They believe ONE.
Answer 2: They believe THAT IT IS OF A TYPE.
The result is an unclear meaning.
Eliminate C.

An OA with believe that:
Many experts believe that policy makers are almost certain to leave interest rates unchanged.
Here, the that-clause in blue correctly serves as the direct object for believe.
Question: WHAT do experts believe?
Answer: They believe THAT POLICY MAKERS ARE ALMOST CERTAIN TO LEAVE INTEREST RATES UNCHANGED.
Since the blue that-clause is the ONLY direct object for believe, the sentence above conveys a logical meaning.