Note that b is true if a is true.
Here we have a straight use of disjunction.
Here is a straightforward example of a conjunction.
This is a hard one. We cannot (yet) write what we want to write: that is, d:- a, (b or c). Here, we can use de Morgan's law: this is the equivalence: a (b c) d (a b) (a c) d.
This is hard too. The obvious solution is:
but this is not allowed. Consequently, we have to transform the expression using the logical equivalence a b b a.