Which of the following options will best complete the relationship given below?
2(101)5
8(246)3
3(???)7

In 2(101)5 and 8(246)3, multiplication of the two digits which are given outside the brackets is written as the first two digits inside the bracket.

The third digit inside the bracket is the addition of the first two digits present inside the bracket

for example,

In 2(101)5 ,

$${2}\times{5}$$ = 10 (which is written as the first two digits of the number which is present inside the bracket)

now, from 10, 1 + 0 = 1 (which is written as the third digit of the number which is present inside the bracket)

In the same way for 3(???)7,

$${3}\times{7}$$ = 21

from 21, 2 + 1 = 3

therefore the answer is 3(213)7