The correct answer is: B. Cracked paints may be cured by removing paint and giving a fresh coat of paint.
A. Blistering may be cured by applying water paint finished with oil paint dried with a little copal varnish. This is not correct. Blistering is caused by moisture getting trapped under the paint. To cure it, you need to remove the damaged paint and allow the surface to dry completely before applying a new coat.
C. Crawling paints may be cured by sand preparing the surface and giving a fresh coat with plenty of turps. This is not correct. Crawling is caused by the paint not adhering properly to the surface. To cure it, you need to sand the surface and prime it before applying a new coat of paint.
D. All of the above. This is not correct. Only option B is correct.
I hope this helps!