Given that
Mass of the stationary man, m = 65 kg
Acceleration of the conveyor belt a = 1 m/s^2
Coefficient of the static friction \mu = 0.2
Acceleration due to gravity a = 9.8 m/s^2

Normal force is balanced by the weight of the man(mg)
N = mg
There is only one force acting on the man, and it is caused by the belt's acceleration.
F_{net} = F
Step 1: Calculating the net force acting on the man
According to the newton's second law of motion
Force, F = ma
F_{net} = ma
F_{net} = 65 kg * 1 m/s^2 = 65 N
Net force acting on the man = 65 N
Step 2: Find the maximum belt acceleration at which a man can remain stationary F_s = \mu N = \mu mg
The man will remain motionless in respect to the conveyor belt until his net force is less than or equal to the belt's frictional force,( F_s ). F_s = \mu N = \mu mg
F_{net} = F_s
ma = \mu mg
a = \mu g
a = 0.2 * 9.8 m/s^2
a = 1.96 m/s^2
Hence, the maximum belt acceleration at which a man can remain stationary a = 1.96 m/s^2