The assignment problem constraint x31 + x32 + x33 + x34 <= 2 means
a. agent 2 can be assigned to at most 3 tasks.
b. agent 3 can be assigned to at most 2 tasks.
c. a mixture of agents 1, 2, 3, and 4 will be assigned to 2 or less tasks.
d. there is no feasible solution.
Assignment problem in stricter sense does not allow this constraint as an agent can be assigned to one and only one task ...
The solution is about interpretation of assignment constraint