Concept:Tayammum is a dry purification using clean earth when water is unavailable or cannot be used.
Explanation:Islam allows Tayammum as a substitute for ablution with water.
It becomes permissible when a Muslim genuinely cannot obtain water, for example in a desert or during severe shortage.
It may also be used when using water would harm one’s health.
The condition is not related to time of prayer, joining congregation, or being in a hurry.
Therefore, among the given options, Tayammum is performed when access to water is very difficult.
Answer:C. access to water is very difficult