What is one effective way for employees to keep their skillsets current?

a. write a blog about personal experiences

b. read journals online or subscribe to them

c. create a large personal network online

d. use social media as often as possible

They are probably looking for B.  subscribing to journals is a good way of keeping up with what's happening in any field.

Of course, a very carefully curated professional network online is great for this too (but they said "personal" so that's not the answer)

Employers generally prefer that employees WORK as often as possible, so even though JUDICIOUS use of social media is a great way to keep current, D isn't the answer either.

Blogging about personal experiences is not necessarily going to teach you anything about work, though blogging professionally can be useful in gathering response from your readership

This statement is correct