Sentence improvement — remove the illogical double cause ('reason' + 'because') and use a clean complement: 'The reason why he wrote the letter was that he could not contact him over the phone.'

Given sentence
The reason why he wrote the letter was because he could not contact him over the phone.

Concept/Approach
Avoid double causatives like 'reason … because'. Standard English uses: 'The reason … was that …' or simply 'He wrote the letter because …'.

Option analysis
• why he wrote the letter was that → correct complementizer 'that' after 'was' avoids redundancy.• 'why … was since' → 'since' is causal but sounds clumsy in this slot.• 'for which … because' → repeats cause ('for which' + 'because'); ungrammatical.• 'No improvement' → retains the redundancy.

Final Answer
why he wrote the letter was that