I had a look and apparently increase in interest rate increases savings through substitution effect, and inflation decreases savings by income effect. Working in the IS-LM case I don't think inflation analysis was possible or required, hence b should be correct.
Also @abhitesh, I believe the question was New Keynesian model price stickiness, amd it so happens that the new keynesian theory also assumes a upward sloped supply curve so we may be wrong.