Step 1: Identify the reporting verb and speech type
The verb "said'' is in the past tense and introduces a statement.
Step 2: Convert present continuous tense to past continuous
The original sentence "I am going...'' (present continuous) changes to "he was going...'' (past continuous).
Step 3: Adjust the pronoun
"I'' becomes "he'' referring to John.
Step 4: Review other options
Option A retains the present tense "is going'' — incorrect.
Option B changes to future "will go'' — incorrect tense shift.
Option D uses the simple present "goes'' — inappropriate for this context.
Conclusion: Option C correctly applies the rules of indirect speech.