The mayan calendar was significant for all of these. held the key to deciphering the hieroglyphic tablets found in palenque. measured time dating back to august of 3114 b. c.e. demonstrating an advanced understanding of astronomy. included a "long count" calendar based on 5,200 years cycles.

The answer is:  [A]  "all of these" .

Note:  "ALL" of the the answer choices provided are correct.

The Mayan calendar was significant for:

 [B]:  "[held]  the key to deciphering the hieroglyphic tablets found in Palenque." 

 [C]:  "[measured] time date back to August of 3114 B.C.E."

{Note:  "B.C.E."  means "Before Common Era".   Same thing as "B.C." (before Christ).  There has, for quite some time been a "movement" to change "B.C." and "A.D."; to:  "B.C.E." ("Before Common Era") and "C.E." (Common Era); respectively.   In practice, both are used.  Just remember:  "B.C.E. = B.C." ; and:  "C.E. = A.D."

[D]:  "[included] a "long count" calendar based on 5,200 years cycles."

{Note:  The Mayans developed and used a "calendar round" and a "Long Count".}.