correct answer is: b) 10%

Explanation

Here ‘a sum’ means ‘principal’.
Let,
Principal = ₹'x'
Rate = r %
Time = 10 years.
So, according the question,
Simple Interest =\frac{p\times t\times r}{100}
\rightarrow\ 2x=\frac{x\times10\times r}{100} _{[put the value]}
\rightarrow\ x\times10\times r=2x\times100 _{[cross multiply]}
\rightarrow\ 10xr=200x
\rightarrow r=\frac{200x}{10x}
\rightarrow\ r = 10
\therefore Rate percent per annum will 10 %
Ans: Rate percent per annum will 10 %.

Another way to phrase this particular query is:
1. What is the annual simple interest rate needed for a sum to double in 10 years?
2. Calculate the percentage rate per annum for simple interest to double a sum in 10 years.
3. At what interest rate per annum will a principal amount double in 10 years with simple interest?
4. Determine the simple interest rate required for a sum to become double within a span of 10 years.
5. Find the annual percentage rate for simple interest that results in a sum doubling in 10 years.