B has the right to rescind the contract. Section 27 of Specific Relief Act, 1963 deals with-When rescission may be adjudged or refused.
Note. – This question is based on Illustration (a) of Section 35, Specific Relief Act, 1877 (1 of 1877).
Illustration (a) to Section 35 is as follows-
Illustration (a): A sells a field to B. There is a right of way over the field of which A has direct personal knowledge, but which he conceals from B. B is entitled to have the contract rescinded.