Final answer:
The Confederate navy secretary called on the construction of ironclad warships to defeat the Union naval blockade. The CSS Florida and the CSS Alabama were examples of these ironclads, though they were ultimately destroyed. Hence, the correct answer is option A.
Step-by-step explanation:
The secret weapon that the Confederate navy secretary called on to be built to defeat the Union naval blockade was ironclad warships. With the blockade in place by the Union, the Confederacy struggled to maintain its maritime activities, including trading with allies such as Great Britain.
Despite the British building and selling ironclad ships to the Confederacy, it wasn't enough to overcome the Union's naval dominance. The Confederacy acquired two ironclads known as the CSS Florida and the CSS Alabama, which were armored to break through the blockade, but both were destroyed during the war. The development and use of ironclad warships during the Civil War marked a significant advancement in naval technology at the time.