Whats the answer O_O

I think its 4, like I'm 90%sure

4. Protein synthesis

Explanation:

structure A is the mitochondria where ATP is produce, also known as protein.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.