Possibly the earlier replies in this and the other referenced thread, do not make clear what VLSM is.
First, we must understand subnetting is subdividing a "natural" classful (Class A implied /8, Class B implied /16 and Class C implied /24) network into subnets. (BTW, supernetting combines natural networks.) Classful networks do not need an explicit network mask, because the address, itself, implies the mask. Subnets of a classful network do need an explicit network mask. (Classless networks all need explicit network masks, and there are, technically, no subnets [or supernets]).
10.0.0.0 is a natural (private Class A) network, so 10.0.0.0/(9..31) is a subnet of it. While 192.168.0.0 is a natural (private Class C), so 192.168.0.0(25..31) is a subnet of it. (192.168.0.0/23 is an example of a supernet of 192.168.0.0 and 192.168.1.0.)
When you subnet a natural network, you can define subnets, all the same size or using the same mask, this is known as FLSM (fixed length subnet mask). An example of FLSM for 192.168.0.0 would be 192.168.0.0/25 and 192.168.0.128/25.
Or, when you subnet a natural network, you can define subnets, of different sizes or using different masks, this is know as VLSM (variable length subnet mask). An example of VLSM for 192.168.0.0 would be 192.168.0.0/26, 192.168.0.64/26 and 192.168.0.128/25.
It's been discussed, that subnetting a subnet is VLSM. Yes and no. In the above VLSM the first /25 was "subnetted" into two /26s, but this is an example of FLSM, if we "subnet" the second /25: 192.168.0.0/26, 192.168.0.64/26, 192.168.0.128/26 and 192.168.0.192/26
The last example looks like the two /25s were subnetted, but actually the original natural (/24) was subnetted into equal size subnets, four /26s
So, subnetting a subnet, isn't the real difference between FLSM and VLSM. You need to determine whether all the natural network's subnets are the same sizes or whether they are variable sizes.
In the OP question, all answers with "and subnetting it to multiple /##s". If that done to a natural network, then it's FLSM. If done to an unnatural network, with an implication it doesn't apply to the parent natural network, it's VLSM.
Consider, in my 192.168.0.0 example, if we subnet the natural 192.168.0.0/24 and all the subnets are the same size, its FLSM, but if we subnet 192.168.0.0/25 into whatever, without stating the same happens to 192.168.0.128/25, it implies we're doing VLSM, but actually, it's unclear what the status is of 192.168.0.128/25 address space. Perhaps we just haven't bothered to state how it will be subdivided. If it too is split into /26s, we have FLSM, if other than two /26s, we have VLSM.
To be really precise, the way the question is worded, with the information that's present, we can only really, truly say, answers B and E are examples of FLSM, while A, C and D may be example of VLSM, but they might not be too.
To summarize:
If ALL the subnets of a (natural/classful) network are the same size, it's FLSM. Otherwise, if ANY the subnets of a (natural/classful) network are different sizes, it's VLSM.
If we're doing classless networking, there's no subnetworks, per se, but we commonly still call breaking one address range into smaller address ranges, subnetting. So, 192.168.0.0/25 might be called a subnet of 192.168.0.0/24 (or 192.168.0.0/23 or 192.168.0.0/22 or . . . 192.168.0.0/16), or just the network 192.168.0.0/25.
