Which is a correctly written sentence, not a run-on?

a.
our cousins are coming for a visit we plan on taking them to the zoo and the museums.

b.
our backyard was all soggy from the rain, the dog's paws were all muddy.

c.
the gate to our backyard had been left open our dogs got out while we were gone.

d.
our car wouldn't start, so dad called a tow truck to take it to the mechanic.