Answer:
1. Option b)
![-2.1 - (5.9+3.7)](https://img.qammunity.org/2020/formulas/mathematics/high-school/gepwhnrl0apk2sm66mginzviyqmpn3a7v1.png)
2. Option b)
![-20 + (-40) + (-60)](https://img.qammunity.org/2020/formulas/mathematics/high-school/t6xeor8uek97avr4h35w7k37dx22ybjh9b.png)
3. Associative property
4. Commutative property
5. -43
Explanation:
1. We are given the expression:
![-2.1 + (-5.9) + (-3.7)](https://img.qammunity.org/2020/formulas/mathematics/high-school/zk4pas071p4etos6lkiyi65mt7s436ckc5.png)
Now, if we take minus common from the last two terms, then, the expression would be of the form:
![-2.1 - (5.9+3.7)](https://img.qammunity.org/2020/formulas/mathematics/high-school/gepwhnrl0apk2sm66mginzviyqmpn3a7v1.png)
Hence, the given expression is equivalent to option b)
![-2.1 - (5.9+3.7)](https://img.qammunity.org/2020/formulas/mathematics/high-school/gepwhnrl0apk2sm66mginzviyqmpn3a7v1.png)
2. We are given the expression:
![-40 + (-20) + (-60)](https://img.qammunity.org/2020/formulas/mathematics/high-school/pdh4a96mdqaswbw75n4vsl4yw37mxf1ng3.png)
Now, if we rearrange the terms in the expression, then, the expression would be of the form:
![-20 + (-40) + (-60)](https://img.qammunity.org/2020/formulas/mathematics/high-school/t6xeor8uek97avr4h35w7k37dx22ybjh9b.png)
Hence, the given expression is equivalent to option b)
![-20 + (-40) + (-60)](https://img.qammunity.org/2020/formulas/mathematics/high-school/t6xeor8uek97avr4h35w7k37dx22ybjh9b.png)
3. The property mentioned here is the associative property.
![a+(b+c) = (a+b)+c](https://img.qammunity.org/2020/formulas/mathematics/high-school/kcmoa7icad77togl0lw44vorz134aax0h1.png)
4. The property mentioned here is the commutative property.
![a+b = b + a](https://img.qammunity.org/2020/formulas/mathematics/high-school/hlopjhugtohq4m66amqhngc0r6q0481m2v.png)
5.
![-35 + 7 + (-15)\\=7 - (35 + 15)\\= 7 - 50\\= -43](https://img.qammunity.org/2020/formulas/mathematics/high-school/b8xktghq78couu2prsg6pgf31x6bi6sj27.png)