What is the difference between a 1st and 2nd Order Digital Filter
I don't have Matlab, but looking at their online help for c2d and c2dOptions:

'FractDelayApproxOrder'
Maximum order of the Thiran filter used to approximate fractional delays in the 'tustin' and 'matched' methods. Takes integer values. A value of 0 means that c2d rounds fractional delays to the nearest integer multiple of the sample time.

So, unless you have a need for a delay, you should set that to zero. The code should look like this:

opt = c2dOptions('Method', 'tustin', 'PrewarpFrequency', 0);
f0 = 80000;
opt = c2dOptions('Method', 'tustin', 'PrewarpFrequency', f0);
LPF_D = c2d(LPF_A, 1/f0, opt);

Let me know if this does it for you (I really don't have Matlab).

Re-read the online description, it looks like 'tustin' allows the 'PrewarpFrequency', so then you should match that with the c2d(), see updated code.