The product obtained when 13 is multiplied by 7(in roman number) is
A.\[{\text{XXCI}}\]
B.\[{\text{LXI}}\]
C.\[{\text{XCI}}\]
D.\[{\text{CI}}\]

Hint: Here, we will first find the product when 13 is multiplied by 7 and then break the above number 91 in the sum of subgroups. Then we will convert each subgroup in the roman numeral and then add back to find the required value.

Complete step-by-step answer:
We are given that the product of 13 with 7 is in roman number.
First, finding the product when 13 is multiplied by 7, we get
\[ \Rightarrow 13 \times 7 = 91\]
Breaking the above number 91 in the sum of subgroups, we get
\[ \Rightarrow 91 = 90 + 1{\text{ ......eq.(1)}}\]
Converting the each subgroup of the above right hand side, we get
\[ \Rightarrow 90 = 100 - 10\]
Since we know 100 is \[{\text{C}}\] and \[10\] is \[{\text{X}}\] in the above equation, we get
\[ \Rightarrow {\text{C - X = XC}}\]
Using the roman numerals of 1 and 90 in the equation \[(1)\], we get
\[ \Rightarrow {\text{XC}} + {\text{I}} = {\text{XCI}}\]
Thus, the required roman numeral is \[{\text{XCI}}\].

Note: In solving these types of questions, students need to know the roman numerals for 1, 10 and 100. We first have to break numbers to find the required value.