Answer
The largest possible value of the fundamental unit of charge is $1.33\times 10^{-19}~C$
Work Step by Step
We can find the difference between each successive charge:
$(3.99\times 10^{-19}~C)-(2.66\times 10^{-19}~C) = 1.33\times 10^{-19}~C$
$(6.65\times 10^{-19}~C)-(3.99\times 10^{-19}~C) = 2.66\times 10^{-19}~C$
$(9.31\times 10^{-19}~C)-(6.65\times 10^{-19}~C) = 2.66\times 10^{-19}~C$
$(10.64\times 10^{-19}~C)-(9.31\times 10^{-19}~C) = 1.33\times 10^{-19}~C$
The largest possible value of the fundamental unit of charge is $1.33\times 10^{-19}~C$
We can see that each charge is a multiple of this value:
$2\times (1.33\times 10^{-19}~C) = 2.66\times 10^{-19}~C$
$3\times (1.33\times 10^{-19}~C) = 3.99\times 10^{-19}~C$
$5\times (1.33\times 10^{-19}~C) = 6.65\times 10^{-19}~C$
$7\times (1.33\times 10^{-19}~C) = 9.31\times 10^{-19}~C$
$8\times (1.33\times 10^{-19}~C) = 10.64\times 10^{-19}~C$