Answer
The maximum rate of deceleration so that the crate does not start sliding is $a = 7.4 ~m/s^2$.
Work Step by Step
The force of static friction between the crate and the truck can accelerate the crate without sliding as long as the rate of acceleration is not too great to overcome the maximum force of static friction.
$ma = F_f$
$ma = mg\mu_s$
$a = g\mu_s$
$a = (9.8 ~m/s^2)(0.75)$
$a = 7.4 ~m/s^2$
The maximum rate of deceleration so that the crate does not start sliding is $a = 7.4 ~m/s^2$.
