The velocity v of a particle in a time t is given by the equation v = 10 + 2t².

Acceleration $a(t)$ is the derivative of velocity $v(t)$ with respect to time $t$:

$$a(t) = \frac{dv}{dt}$$

Given $v(t) = 10 + 2t^2$:

$$a(t) = \frac{d}{dt}(10 + 2t^2)$$ $$a(t) = \frac{d}{dt}(10) + \frac{d}{dt}(2t^2)$$ $$a(t) = 0 + 2(2t^{2-1})$$ $$a(t) = 4t$$

Substitute $t = 5$ into the acceleration function:

$$a(5) = 4(5)$$ $$a(5) = 20\text{m/s}^2$$

Comparing this result with the given options:
A. $60\text{m/s}^2$
B. $15\text{m/s}^2$
C. $20\text{m/s}^2$
D. $10\text{m/s}^2$

The calculated instantaneous acceleration matches option C.

The final answer is \boxed{\text{C. *20 m/s^2}}.