Step 1: Understanding the Concept:
Estrus is characterized by mucus discharge.
The mucus is produced by specific cells.
Step 2: Key Formula or Approach:
No specific formula.
Need to know the source of estrus mucus.
Step 3: Detailed Explanation:
The mucus seen at estrus is the secretion of goblet cells of the cervix.
Cervical mucus changes during the estrous cycle.
Endometrium, myometrium, and vagina do not produce this mucus.
Thus, the mucus is from the cervix.
Step 4: Final Answer:
The mucus at estrus is secreted by goblet cells of the cervix.
Hence, the correct option is (A).