The best practices for writing triggers in Salesforce include bulkifying your code to handle multiple records efficiently and reducing the number of SOQL queries, especially within loops, to avoid hitting governor limits.

Option B is correct because moving the SOQL query outside of the for loop is essential to bulkify the trigger. Performing a query within a loop can cause a SOQL governor limit to be reached if the trigger is processing many records. The best practice is to collect all necessary data before the loop starts and then process it within the loop.

Option C is correct as well, because using a Map to cache the results of a SOQL query by Id is another way to bulkify the code. This allows you to query all the necessary data once and then access it efficiently in memory, avoiding repeated queries.

Option A is incorrect because adding isExecuting before the update doesn't address any best practices related to bulkification or SOQL query optimization.

Option D is incorrect because the last line should not be removed if the developer intends to perform DML on the accountList. However, this line should be outside of the loop and is unnecessary if the trigger context is 'before insert' or 'before update', since in those contexts, changes to records in Trigger.new are implicitly saved without the need for explicit DML.