A lot of debate going on the Facebook post for this.
To me, it was certainly 'in'. But I may consider 'at' as well. I'm confused!
Why 'in'? Because you do degrees 'in' some subject/topic.
Why 'at'? Because then you consider 'specializing....' as another clause supporting the 'law degree'. So, you specialize 'at' something!
The correct answer is in.
For that example, the answer is definitely in, nothing else there would be even slightly correct. In English, the preposition relating to work is usually in, including for your second example with specialise.
For example:
I work in engineering.
I specialise in civil engineering.
In sentences like "I work at a bar", the preposition relates to the location of work, not the profession itself.
