Final answer:
Deserts were the most difficult physical barrier to overcome with the spread of Islam.
Step-by-step explanation:
The most difficult physical barrier to overcome with the spread of Islam was deserts. Deserts, such as the Arabian Desert, posed significant challenges for early Muslim traders and armies. These arid environments lacked water and vegetation, making travel and sustenance difficult. The desert's harsh conditions necessitated adaptions such as the establishment of oasis trading posts and camel caravans.
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