In the Arecibo Message Shown Above, the Purple Line Leads

In the Arecibo message shown above, the purple line leads to a purple arc with a letter M, which represents ______________.

A) a common color of the rainbow being transmitted in a region of the EM spectrum that text messages are sent in
B) two letters of the Greek alphabet
C) the radio dish sending a common letter of the English alphabet
D) the radio dish sending a signal