The Contractor is entitled to an advance payment.

Comprehensive and Detailed Explanation:

Under FIDIC Red Book 1999, advance payment is intended to assist the Contractor in financing the mobilization and early works costs before the Contractor begins receiving regular payments for work performed (Option A). It is not a payment for work already completed, so Option B is incorrect.

The General Conditions provide that the advance payment is usually made in a lump sum or agreed instalments and that the Employer makes the payment in advance (Option C is partially true but generally it can be one or multiple instalments, depending on contract terms).

Importantly, the advance payment must be repaid by the Contractor through deductions from subsequent interim payments (Option D), ensuring the Employer recovers the advanced funds as the work progresses.