Thank you for the opportunity to help you with your question!
Cross Sectional- When we conduct a study using cross-sectional design, we take a group of samples from a set, or continuum, to see if there are any differences in the section of the continuum. Cross-sectional designs, defined as sampled groups along a developmental path in an experiment to determine how development influences a research variable. Let's say your three age groups are 20-35 years, 36-50 years, and 51-65 years. You can then calculate the percentage of women in each group that have been diagnosed with breast cancer. This information can then be used to answer your question.This is an example of cross-sectional research. Cross-sectional research involves using different groups of people who differ in the variable of interest but share other characteristics, such as socioeconomic status, educational background, and ethnicity.
Longitudinal- Longitudinal designs is a research study where a sample of the population is studied at intervals to examine the effects of development. To conduct a longitudinal design, we start with 20-year-olds, and then check in with them every 20 years to see how they've changed.
Cross- sequential- It is defined as a combination of longitudinal and cross-sectional designs, by following several differently aged cohorts over time. With longitudinal, we look at one group over a long time. With cross-sectional, we look at a whole bunch of groups right now. With sequential, we look at a whole bunch of groups over time.
Second part- For this research, the most suited method will be Longitudinal as the habit of nail biting can be observed over a time period. It can be observed on the sample at different intervals. Medical experts are now taking a closer look at the addiction and have decided to change its classification from a mere habit to a full-fledged obsessive-compulsive disorder.
Please let me know if you need any clarification. I'm always happy to answer your questions.