Which of the following sets of numbers could be the lengths of the sides of a triangle? a. 81 mm, 7 mm, 6 mm b. 81 mm, 7 mm, 72 mm c. 81 mm, 7 mm, 88 mm d. 81 mm, 7 mm, 77 mm

We know that
The triangle inequality states that for any triangle, the sum of the lengths of any two sides of a triangle is greater than the length of the third side
so
case A.81 mm, 7 mm, 6 mm 
6+7 is not > 81

case B.81 mm, 7 mm, 72 mm
72+7 is not > 81

case C.81 mm, 7 mm, 88 mm
81+7 is not > 88

case D.81 mm, 7 mm, 77 mm
81+7 is > 77> ok
77+7 is > 81> ok
81+77 is > 7> is ok

the answer is the option
D.81 mm, 7 mm, 77 mm