Should we have a sub domain, a new domain or a different path for new a category?

We have a platform Skill-Guru for teachers and experts to create test.

The Skill-Guru blog section has topics which are mostly technical and some new news or test announcement .

Now we have experts on English language , grammar , SAT , GMAT who would be creating tests on our site.
We would also like to give the readers some content to read apart from test.
So I was thinking what is the best choice in terms of
a) Search engine optimization perspective
b) Customer convenience -

We do not want our technical readers to read articles about GMAt and SAT when they expect technical topics. So each specific topic should have blog dedicated to itself.

The options we were considering
a) have a new domain for each category and host blog in the root directory of new domain for eg

english-skill-guru.com

gmat-skill-guru.com

Problem - each site will have touch time getting to same ranks as parent site.

b) have sub domain within main domain so that would be helpful form SEO perspective for eg

english.skill-guru.com

gmat.english-guru.com
Here each sub domain will have their own blog hosted.

c) have a different url for each category (technically it means that have a new blog installed in new directory) for eg

www.skill-guru.com/english/

www.skill-guru.com/GMAT/

I am not sure if overall this approach is correct or not.
Advice appreciated.

Google cares a lot about how close your keyword is to the domain name. All else being equal, gmat-skill-guru.com is better than skill-guru.com/gmat which is better than skill-guru.com/blog/gmat. Google treats sub-domains as if they appear just after the slash, so gmat.skill-guru.com is equivalent to skill-guru.com/gmat.

So the question is really whether or not to have a separate domain name (gmat-skill-guru.com). Having a separate domain gets your keyword into the domain, which is good. But it also dilutes your backlinks, since incoming links will be distributed across lots of different sites instead of being concentrated on one skill-guru.com site. You need to decide which you care about more - the SEO juice that comes from backlinks or the SEO juice that comes from having your keyword in the domain name.

Google is reducing the weight of backlinks when determining a site's relevance, so the fact that you will be diluting pagerank is not as big a deal as it used to be. It still matters, but not as much. And Google is signaling that it will matter even less in the future.

I think that I would use different domain names, and here's why. In the future, if one of your sites takes off, it could become an acquisition target. For example, if your GMAT site gets a lot of traffic, Kaplan might want to buy it from you. It will be easy to sell that one site if it's stand-alone, but if it's a section of a larger site, it's much harder.

I would say option B is best practice these days and probably the easiest to implement for separate blogs. You can do some value add things as well by being able to share cookies between the sub-domains, which wouldn't be possible with option a.

Best of Luck, Doug

Well isn't the sub domain only important for those who already know about the site?

Won't those searching for "X test" get your parent site anyway if they search?