What do you do if you see your team is taking assigned job easy?

To help people take more responsibility for their work, provide them with the skills and resources to actually do their job.
Then set up an environment that makes it easy for them to change, and help them take responsibility for their decisions and actions.This can be done by ensuring adequate resources, Communicating roles, responsibilities, and objectives., Re-engaging them, Helping them take control, Avoiding micromanagement and most importantly giving praise.

Warn the team in advance of further consequences if the objective is not met.

i think motivation plus authority will be the key to go.
As motivation will boost there interest in the task and authority will make them feel a sense of responsibility.
divide the task and give it as a responsibility to all individuals and then let them work in a team.
when they will know that they are answer able to there participation individually they will surely take it seriously.

Although the task is very easy to be accepted by the team, the most important task of how to responsibilities that had received it.

I think if I am the manager I will assign the job to each member and be sure I put the correct guy in correct place

In my opinion, management may like to introduce incentive schemes for a good, better, best performer,that will infuse competition and all the team members will be charged.
One example can be as many companies award best performer of the month, put up a board in their department and write their name.
This really motivates him/her to retain their name on the top next month also and motivates others to replace his/her name with theirs.