What is one reason document a would be a reliable source to understand what happened when moctezuma met cortes?

The reason is because Document A is the primary source written to the king.

Explanation:

Document A is said to be reliable source which helped to understand what had actually happened when Moctezuma had met Cortes. This was considered to be a reliable source because this was the primary source which was written to the king Charles V.

Because it was a primary source, it was obviously a reliable source. This document was a letter written to the king which described the meeting of the two.

d. direct

the  seven  days  battles  were  a series of seven battles over seven  days from june 25 to july 1, 1862, near richmond, virginia,  during  the american civil war. the  seven  days ended with mcclellan's army in relative safety next to the james river, having suffered almost 16,000 casualtiesduring  the retreat.