Answer: D.
![7t^2+3+6](https://img.qammunity.org/2020/formulas/mathematics/college/5nv97ekkf6bamrmc4nzu8u5jmofc91xksx.png)
Explanation:
Given: The function 'a' represents the cost of manufacturing product A, in hundreds of dollars, and the function 'b' represents the cost of manufacturing product B, in hundreds of dollars.
![a(t)=5t+2\\\\b(t)=7t^2-2t+4](https://img.qammunity.org/2020/formulas/mathematics/college/g9w5qa33sq19r4mkukn142p8lb4xmptjft.png)
The expression that describes the total cost of manufacturing both products will be ,
![a(t) + b(t)=5t+2+7t^2-2t+4](https://img.qammunity.org/2020/formulas/mathematics/college/j0bvez8362dwuy8cxlp0ik8ot75fucrnr3.png)
Combining like terms, we get
![a(t) + b(t)=7t^2+5t-2t+4+2\\\\\Rightarrow\ a(t) + b(t)=7t^2+3+6](https://img.qammunity.org/2020/formulas/mathematics/college/vzcj9d1s751m0353mke3zkjezi5gfgmps7.png)