Solution 4 W2 or “a” are the most common ways to build center thickness, and W“c” or “d” are sometimes preferable, all in accord with the principle stated earlier of avoiding contact.
W“b” may be chosen when White is more interested in the lower left than the center.
The appealing looking W”e” is actually a mistake because it’s too close to the Black stone!
After the natural splitting response of B“f”, White will have two weak groups to manage, and will almost certainly end in trouble on one side or the other.