Answer:
The best way to combine the sentences is "When bricking a building, masons must fit the bricks together; Sometimes, they slice bricks in half for a better fit."
Step-by-step explanation:
The best way to combine the sentences presents the use of commas and a semicolon.
The first comma follows the rule of "Use a comma after a dependent clause that starts a sentence", then "When bricking a building," is the depending clause.
The semicolon follows the rule "to link (in a single sentence) two independent clauses that are closely related in thought" both sentences are talking about the same process, then "... the bricks together; Sometimes, they slice bricks..." is the merge of the two sentences into one.
The second comma follows the rule "Use a comma after introductory adverbs". Sometimes is a frequency adverb, then "Sometimes, they..."