What would you consider the below sampling method to be in this description and why?

Participants were recruited via newspapers, online platforms and posters. They filled in online forms assessing demographic variables, health and sleep habits. Inclusion criteria were being 18-30 or 60-72 years old, fluent in Swedish and living in the greater Stockholm area. Further criteria were a BMI between 17 and 28 (self-reported), no present or past psychiatric or neurological disorder (self-reported complaints), no current severe somatic disease. Individuals that took medication that likely affects the stress response, cognition or sleep (e.g. antidepressants, systemic beta blockers, angiotensin II receptor blockers, hormone replacement therapy, hormonal contraceptives), that reported daily use of nicotine, drinking more than 4 cups of caffeinated drinks/day or reported inability to abstain from caffeine were excluded. Moreover, participants were screened for insomnia (Insomnia Severity Index (Morin et al., 2011) > 14), daytime sleepiness (sleepiness index of Karolinska Sleep Questionnaire 3), suspicion of sleep apnea (snoring index of Karolinska Sleep Questionnaire 3 or self-reported sleep apnea), current severe stress (self-reported) or burnout (Shirom Melamed Burnout Questionnaire 3.75), anxiety and depression symptoms (Hospital Anxiety and Depression Scale (Zigmond and Snaith, 1983); >10 for each scale). Further exclusion criteria were night work or travel across more than 1 time zone during the previous month, inability to find time to participate, and (for women) being pregnant or breastfeeding. 21 young women in the TSST, and 17 young women in the Placebo-TSST conditions were tested during the luteal phase (defined as 12 days or less to expected onset of next menstruation). For 2 women in the TSST, and 7 women in the Placebo-TSST condition menstrual phase was likely not luteal; for 4 women information was missing due to technical error. 75.53% of the older sample were retired, 11.7% part time retired, and 12.77% not retired. The proportion did not significantly vary across sleep deprivation / stress conditions (X² (df=6)=4.09, p=0.67).

Participants that fulfilled the inclusion criteria were invited to an initial meeting in the laboratory approximately one week before the test session. During this meeting, Mini Mental State Examination (Folstein et al., 1975) was conducted; participants with a score <24 were excluded. Moreover, participants received sleep diaries and actigraphs. Participants were economically compensated. All participants gave written informed consent. The study was approved by the Regional Ethical Review Board in Stockholm.

A - simple random sampling,

B - systematic sampling,

C - stratified random sampling,

D -proportionate stratified sampling, and

E - cluster sampling

Please explain why. you picked that sample, I am having a hard time figuring this out, Thank you!