The choices are:
Observational study: prospective
Observational study: retrospective
Observational study: cross-sectional
Experiment: matched pairs design
Experiment: randomized block design
Experiment: completely randomized design

I am very confused in my understanding of the different experiment types. Please help!

Read this page and see if it comes clearer --

Experimental Design (http://stattrek.com/experiments/experimental-design.aspx)