How does methyl vinyl ketone dimerise with heating?
The dimer of methyl vinyl ketone (1) is not shown in the choices in the question. The dimer is A shown below. It is a "tail-to-tail" dimer. Product B is an intramolecular ketal as a probable mixture of epimers. Compare with this paper. 



B. P. Mundy, et al., J. Org. Chem., 1978, 43, 2347.