Given:
To round off to the nearest tenth of the given numbers and identify the underestimate term.
Consider the first term.
![\begin{gathered} 39.45*1.7 \\ =67.065 \\ \approx67.1 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/l0l7bkvfcgazuhzzbfd7qanp4hh97jhta5.png)
Consider the second term.
![\begin{gathered} 27.54*0.74 \\ =20.3796 \\ \approx20.4 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/udjcdsk8nsd2ojx8h5pcjrt1408d7rimth.png)
Consider the third term.
![\begin{gathered} 9.91*8.74 \\ =86.6134 \\ \approx86.6 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/on9beuct9of3jgiwyiv9xiyw92ug897yg8.png)
Consider the fourth term.
![\begin{gathered} 78.95*1.26 \\ =99.477 \\ \approx99.5 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/b80tetksq9t3hxvbrj7r6kok4l8vxl0wjr.png)
Consider the fifth term.
![\begin{gathered} 18.19*2.28 \\ =41.4732 \\ \approx41.5 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/rnidljpa8huwekabsfkh6nze6wbj8tra5r.png)
Thus, the obtained results are 67.1, 20.4, 86.6, 99.5, 41.5.
By comparing the results the underestimate term will be 20.4
Hence, option (b) is the correct answer.