What type of reaction is shown below?

Ca + 2H2O - Ca (OH) 2 + H2

A + BC - --> AB + C

So here one reactant (A) is accepting a group which is being given by another compound (BC) however the A is not giving any group / element or ion

So this single displacement

Similarly in the given reaction

the anion OH - is only being replaced

The element Ca accepts OH - and H2O loses the same group to form new element H2

So the correct answer is

Single replacement also known as single displacement.