The 20 days for serving a preliminary notice run from the date the claimant:

解析

Civil Code §8204(a) requires the preliminary notice to be given not later than 20 days after the claimant has first furnished work on the work of improvement, and a claimant who gives it later may claim only for work provided within the 20 days before service and afterwards. (d) is the most attractive wrong answer, since a subcontract is often signed weeks before anyone mobilizes and the statute counts furnishing rather than signing. (c) reverses the purpose of the notice, which is served before there is any payment problem. (a) confuses the two instruments: the notice comes first, and §8410 makes it a condition of enforcing the lien that is recorded later.